diff --git a/goldens/public-api/core/core.d.ts b/goldens/public-api/core/core.d.ts
index ad6dd6a01e..115a93cfbd 100644
--- a/goldens/public-api/core/core.d.ts
+++ b/goldens/public-api/core/core.d.ts
@@ -838,11 +838,11 @@ export declare interface RendererType2 {
 }
 
 export declare class ResolvedReflectiveFactory {
-    dependencies: ɵangular_packages_core_core_d[];
+    dependencies: ɵangular_packages_core_core_e[];
     factory: Function;
     constructor(
     factory: Function,
-    dependencies: ɵangular_packages_core_core_d[]);
+    dependencies: ɵangular_packages_core_core_e[]);
 }
 
 export declare interface ResolvedReflectiveProvider {
diff --git a/packages/core/src/di/injector_compatibility.ts b/packages/core/src/di/injector_compatibility.ts
index 3332f92dfe..8dc6e32116 100644
--- a/packages/core/src/di/injector_compatibility.ts
+++ b/packages/core/src/di/injector_compatibility.ts
@@ -16,14 +16,20 @@ import {resolveForwardRef} from './forward_ref';
 import {getInjectImplementation, injectRootLimpMode} from './inject_switch';
 import {InjectionToken} from './injection_token';
 import {Injector} from './injector';
-import {InjectFlags} from './interface/injector';
+import {DecoratorFlags, InjectFlags, InternalInjectFlags} from './interface/injector';
 import {ValueProvider} from './interface/provider';
-import {Host, Inject, Optional, Self, SkipSelf} from './metadata';
 
 
 const _THROW_IF_NOT_FOUND = {};
 export const THROW_IF_NOT_FOUND = _THROW_IF_NOT_FOUND;
 
+/*
+ * Name of a property (that we patch onto DI decorator), which is used as an annotation of which
+ * InjectFlag this decorator represents. This allows to avoid direct references to the DI decorators
+ * in the code, thus making them tree-shakable.
+ */
+const DI_DECORATOR_FLAG = '__NG_DI_FLAG__';
+
 export const NG_TEMP_TOKEN_PATH = 'ngTempTokenPath';
 const NG_TOKEN_PATH = 'ngTokenPath';
 const NEW_LINE = /\n/gm;
@@ -145,17 +151,14 @@ export function injectArgs(types: (Type<any>|InjectionToken<any>|any[])[]): any[
 
       for (let j = 0; j < arg.length; j++) {
         const meta = arg[j];
-        if (meta instanceof Optional || meta.ngMetadataName === 'Optional' || meta === Optional) {
-          flags |= InjectFlags.Optional;
-        } else if (
-            meta instanceof SkipSelf || meta.ngMetadataName === 'SkipSelf' || meta === SkipSelf) {
-          flags |= InjectFlags.SkipSelf;
-        } else if (meta instanceof Self || meta.ngMetadataName === 'Self' || meta === Self) {
-          flags |= InjectFlags.Self;
-        } else if (meta instanceof Host || meta.ngMetadataName === 'Host' || meta === Host) {
-          flags |= InjectFlags.Host;
-        } else if (meta instanceof Inject || meta === Inject) {
-          type = meta.token;
+        const flag = getInjectFlag(meta);
+        if (typeof flag === 'number') {
+          // Special case when we handle @Inject decorator.
+          if (flag === DecoratorFlags.Inject) {
+            type = meta.token;
+          } else {
+            flags |= flag;
+          }
         } else {
           type = meta;
         }
@@ -169,6 +172,30 @@ export function injectArgs(types: (Type<any>|InjectionToken<any>|any[])[]): any[
   return args;
 }
 
+/**
+ * Attaches a given InjectFlag to a given decorator using monkey-patching.
+ * Since DI decorators can be used in providers `deps` array (when provider is configured using
+ * `useFactory`) without initialization (e.g. `Host`) and as an instance (e.g. `new Host()`), we
+ * attach the flag to make it available both as a static property and as a field on decorator
+ * instance.
+ *
+ * @param decorator Provided DI decorator.
+ * @param flag InjectFlag that should be applied.
+ */
+export function attachInjectFlag(decorator: any, flag: InternalInjectFlags|DecoratorFlags): any {
+  decorator[DI_DECORATOR_FLAG] = flag;
+  decorator.prototype[DI_DECORATOR_FLAG] = flag;
+  return decorator;
+}
+
+/**
+ * Reads monkey-patched property that contains InjectFlag attached to a decorator.
+ *
+ * @param token Token that may contain monkey-patched DI flags property.
+ */
+export function getInjectFlag(token: any): number|undefined {
+  return token[DI_DECORATOR_FLAG];
+}
 
 export function catchInjectorError(
     e: any, token: any, injectorErrorName: string, source: string|null): never {
diff --git a/packages/core/src/di/interface/injector.ts b/packages/core/src/di/interface/injector.ts
index 9092ef5deb..7d11dd5980 100644
--- a/packages/core/src/di/interface/injector.ts
+++ b/packages/core/src/di/interface/injector.ts
@@ -7,25 +7,67 @@
  */
 
 
+/**
+ * Special flag indicating that a decorator is of type `Inject`. It's used to make `Inject`
+ * decorator tree-shakable (so we don't have to rely on the `instanceof` checks).
+ * Note: this flag is not included into the `InjectFlags` since it's an internal-only API.
+ */
+export const enum DecoratorFlags {
+  Inject = -1
+}
+
 /**
  * Injection flags for DI.
  *
  * @publicApi
  */
 export enum InjectFlags {
-  // TODO(alxhub): make this 'const' when ngc no longer writes exports of it into ngfactory files.
+  // TODO(alxhub): make this 'const' (and remove `InternalInjectFlags` enum) when ngc no longer
+  // writes exports of it into ngfactory files.
 
   /** Check self and check parent injector if needed */
   Default = 0b0000,
+
   /**
    * Specifies that an injector should retrieve a dependency from any injector until reaching the
    * host element of the current component. (Only used with Element Injector)
    */
   Host = 0b0001,
+
   /** Don't ascend to ancestors of the node requesting injection. */
   Self = 0b0010,
+
   /** Skip the node that is requesting injection. */
   SkipSelf = 0b0100,
+
   /** Inject `defaultValue` instead if token not found. */
   Optional = 0b1000,
 }
+
+/**
+ * This enum is an exact copy of the `InjectFlags` enum above, but the difference is that this is a
+ * const enum, so actual enum values would be inlined in generated code. The `InjectFlags` enum can
+ * be turned into a const enum when ViewEngine is removed (see TODO at the `InjectFlags` enum
+ * above). The benefit of inlining is that we can use these flags at the top level without affecting
+ * tree-shaking (see "no-toplevel-property-access" tslint rule for more info).
+ * Keep this enum in sync with `InjectFlags` enum above.
+ */
+export const enum InternalInjectFlags {
+  /** Check self and check parent injector if needed */
+  Default = 0b0000,
+
+  /**
+   * Specifies that an injector should retrieve a dependency from any injector until reaching the
+   * host element of the current component. (Only used with Element Injector)
+   */
+  Host = 0b0001,
+
+  /** Don't ascend to ancestors of the node requesting injection. */
+  Self = 0b0010,
+
+  /** Skip the node that is requesting injection. */
+  SkipSelf = 0b0100,
+
+  /** Inject `defaultValue` instead if token not found. */
+  Optional = 0b1000,
+}
\ No newline at end of file
diff --git a/packages/core/src/di/metadata.ts b/packages/core/src/di/metadata.ts
index d3e9275f03..913f3e8ce8 100644
--- a/packages/core/src/di/metadata.ts
+++ b/packages/core/src/di/metadata.ts
@@ -8,6 +8,9 @@
 
 import {makeParamDecorator} from '../util/decorators';
 
+import {attachInjectFlag} from './injector_compatibility';
+import {DecoratorFlags, InternalInjectFlags} from './interface/injector';
+
 
 /**
  * Type of the Inject decorator / constructor function.
@@ -54,8 +57,10 @@ export interface Inject {
  * @Annotation
  * @publicApi
  */
-export const Inject: InjectDecorator = makeParamDecorator('Inject', (token: any) => ({token}));
-
+export const Inject: InjectDecorator = attachInjectFlag(
+    // Disable tslint because `DecoratorFlags` is a const enum which gets inlined.
+    // tslint:disable-next-line: no-toplevel-property-access
+    makeParamDecorator('Inject', (token: any) => ({token})), DecoratorFlags.Inject);
 
 /**
  * Type of the Optional decorator / constructor function.
@@ -97,7 +102,10 @@ export interface Optional {}
  * @Annotation
  * @publicApi
  */
-export const Optional: OptionalDecorator = makeParamDecorator('Optional');
+export const Optional: OptionalDecorator =
+    // Disable tslint because `InternalInjectFlags` is a const enum which gets inlined.
+    // tslint:disable-next-line: no-toplevel-property-access
+    attachInjectFlag(makeParamDecorator('Optional'), InternalInjectFlags.Optional);
 
 /**
  * Type of the Self decorator / constructor function.
@@ -142,7 +150,10 @@ export interface Self {}
  * @Annotation
  * @publicApi
  */
-export const Self: SelfDecorator = makeParamDecorator('Self');
+export const Self: SelfDecorator =
+    // Disable tslint because `InternalInjectFlags` is a const enum which gets inlined.
+    // tslint:disable-next-line: no-toplevel-property-access
+    attachInjectFlag(makeParamDecorator('Self'), InternalInjectFlags.Self);
 
 
 /**
@@ -187,7 +198,10 @@ export interface SkipSelf {}
  * @Annotation
  * @publicApi
  */
-export const SkipSelf: SkipSelfDecorator = makeParamDecorator('SkipSelf');
+export const SkipSelf: SkipSelfDecorator =
+    // Disable tslint because `InternalInjectFlags` is a const enum which gets inlined.
+    // tslint:disable-next-line: no-toplevel-property-access
+    attachInjectFlag(makeParamDecorator('SkipSelf'), InternalInjectFlags.SkipSelf);
 
 /**
  * Type of the `Host` decorator / constructor function.
@@ -227,4 +241,7 @@ export interface Host {}
  * @Annotation
  * @publicApi
  */
-export const Host: HostDecorator = makeParamDecorator('Host');
+export const Host: HostDecorator =
+    // Disable tslint because `InternalInjectFlags` is a const enum which gets inlined.
+    // tslint:disable-next-line: no-toplevel-property-access
+    attachInjectFlag(makeParamDecorator('Host'), InternalInjectFlags.Host);
\ No newline at end of file
diff --git a/packages/core/test/bundling/forms/bundle.golden_symbols.json b/packages/core/test/bundling/forms/bundle.golden_symbols.json
index a4d53d02c5..8ef0dcdff9 100644
--- a/packages/core/test/bundling/forms/bundle.golden_symbols.json
+++ b/packages/core/test/bundling/forms/bundle.golden_symbols.json
@@ -236,9 +236,6 @@
   {
     "name": "FormsModule"
   },
-  {
-    "name": "Host"
-  },
   {
     "name": "INJECTOR"
   },
@@ -569,9 +566,6 @@
   {
     "name": "SelectMultipleControlValueAccessor"
   },
-  {
-    "name": "Self"
-  },
   {
     "name": "ShadowDomRenderer"
   },
@@ -743,6 +737,9 @@
   {
     "name": "applyView"
   },
+  {
+    "name": "attachInjectFlag"
+  },
   {
     "name": "attachPatchData"
   },
diff --git a/packages/core/test/bundling/injection/bundle.golden_symbols.json b/packages/core/test/bundling/injection/bundle.golden_symbols.json
index 45532497ed..951bb4b9af 100644
--- a/packages/core/test/bundling/injection/bundle.golden_symbols.json
+++ b/packages/core/test/bundling/injection/bundle.golden_symbols.json
@@ -5,18 +5,12 @@
   {
     "name": "EMPTY_ARRAY"
   },
-  {
-    "name": "Host"
-  },
   {
     "name": "INJECTOR"
   },
   {
     "name": "INJECTOR_SCOPE"
   },
-  {
-    "name": "Inject"
-  },
   {
     "name": "InjectFlags"
   },
@@ -50,21 +44,12 @@
   {
     "name": "NullInjector"
   },
-  {
-    "name": "Optional"
-  },
   {
     "name": "R3Injector"
   },
   {
     "name": "ScopedService"
   },
-  {
-    "name": "Self"
-  },
-  {
-    "name": "SkipSelf"
-  },
   {
     "name": "THROW_IF_NOT_FOUND"
   },
@@ -113,9 +98,6 @@
   {
     "name": "isValueProvider"
   },
-  {
-    "name": "makeParamDecorator"
-  },
   {
     "name": "makeRecord"
   },
diff --git a/packages/core/test/bundling/router/bundle.golden_symbols.json b/packages/core/test/bundling/router/bundle.golden_symbols.json
index fb4ba73571..02cdefc89a 100644
--- a/packages/core/test/bundling/router/bundle.golden_symbols.json
+++ b/packages/core/test/bundling/router/bundle.golden_symbols.json
@@ -290,9 +290,6 @@
   {
     "name": "HashLocationStrategy"
   },
-  {
-    "name": "Host"
-  },
   {
     "name": "INITIAL_VALUE"
   },
@@ -731,9 +728,6 @@
   {
     "name": "SecurityContext"
   },
-  {
-    "name": "Self"
-  },
   {
     "name": "ShadowDomRenderer"
   },
@@ -971,6 +965,9 @@
   {
     "name": "applyView"
   },
+  {
+    "name": "attachInjectFlag"
+  },
   {
     "name": "attachPatchData"
   },
diff --git a/packages/core/test/bundling/todo/bundle.golden_symbols.json b/packages/core/test/bundling/todo/bundle.golden_symbols.json
index 2062b31ee3..86bd5b20cf 100644
--- a/packages/core/test/bundling/todo/bundle.golden_symbols.json
+++ b/packages/core/test/bundling/todo/bundle.golden_symbols.json
@@ -221,6 +221,9 @@
   {
     "name": "assertTemplate"
   },
+  {
+    "name": "attachInjectFlag"
+  },
   {
     "name": "attachPatchData"
   },
diff --git a/packages/core/test/di/inject_flags_spec.ts b/packages/core/test/di/inject_flags_spec.ts
new file mode 100644
index 0000000000..b680ff8c5f
--- /dev/null
+++ b/packages/core/test/di/inject_flags_spec.ts
@@ -0,0 +1,19 @@
+/**
+ * @license
+ * Copyright Google LLC All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.io/license
+ */
+
+import {InjectFlags, InternalInjectFlags} from '../../src/di/interface/injector';
+
+describe('InjectFlags', () => {
+  it('should always match InternalInjectFlags', () => {
+    expect(InjectFlags.Default).toEqual(InternalInjectFlags.Default as number);
+    expect(InjectFlags.Host).toEqual(InternalInjectFlags.Host as number);
+    expect(InjectFlags.Self).toEqual(InternalInjectFlags.Self as number);
+    expect(InjectFlags.SkipSelf).toEqual(InternalInjectFlags.SkipSelf as number);
+    expect(InjectFlags.Optional).toEqual(InternalInjectFlags.Optional as number);
+  });
+});
\ No newline at end of file