diff --git a/integration/_payload-limits.json b/integration/_payload-limits.json
index 9d62c9f0fa..9957587800 100644
--- a/integration/_payload-limits.json
+++ b/integration/_payload-limits.json
@@ -3,7 +3,7 @@
     "master": {
       "uncompressed": {
         "runtime": 1497,
-        "main": 166739,
+        "main": 166799,
         "polyfills": 43626
       }
     }
@@ -34,4 +34,4 @@
       }
     }
   }
-}
+}
\ No newline at end of file
diff --git a/integration/side-effects/snapshots/core/esm2015.js b/integration/side-effects/snapshots/core/esm2015.js
index 458c867642..7e58a587f5 100644
--- a/integration/side-effects/snapshots/core/esm2015.js
+++ b/integration/side-effects/snapshots/core/esm2015.js
@@ -30,20 +30,16 @@ function getSymbolIterator() {
 
 if ("undefined" === typeof ngI18nClosureMode) _global["ngI18nClosureMode"] = "undefined" !== typeof goog && "function" === typeof goog.getMsg;
 
-function flatten(list, mapFn) {
-    const result = [];
-    let i = 0;
-    while (i < list.length) {
-        const item = list[i];
-        if (Array.isArray(item)) if (item.length > 0) {
-            list = item.concat(list.slice(i + 1));
-            i = 0;
-        } else i++; else {
-            result.push(mapFn ? mapFn(item) : item);
-            i++;
-        }
+function flatten(list, dst) {
+    if (void 0 === dst) dst = list;
+    for (let i = 0; i < list.length; i++) {
+        let item = list[i];
+        if (Array.isArray(item)) {
+            if (dst === list) dst = list.slice(0, i);
+            flatten(item, dst);
+        } else if (dst !== list) dst.push(item);
     }
-    return result;
+    return dst;
 }
 
 class EventEmitter extends Subject {
diff --git a/packages/core/src/render3/jit/module.ts b/packages/core/src/render3/jit/module.ts
index 4d35a0ec89..fe41493869 100644
--- a/packages/core/src/render3/jit/module.ts
+++ b/packages/core/src/render3/jit/module.ts
@@ -108,11 +108,13 @@ export function compileNgModuleDefs(moduleType: NgModuleType, ngModule: NgModule
         ngModuleDef = getCompilerFacade().compileNgModule(
             angularCoreEnv, `ng:///${moduleType.name}/ngModuleDef.js`, {
               type: moduleType,
-              bootstrap: flatten(ngModule.bootstrap || EMPTY_ARRAY, resolveForwardRef),
+              bootstrap: flatten(ngModule.bootstrap || EMPTY_ARRAY).map(resolveForwardRef),
               declarations: declarations.map(resolveForwardRef),
-              imports: flatten(ngModule.imports || EMPTY_ARRAY, resolveForwardRef)
+              imports: flatten(ngModule.imports || EMPTY_ARRAY)
+                           .map(resolveForwardRef)
                            .map(expandModuleWithProviders),
-              exports: flatten(ngModule.exports || EMPTY_ARRAY, resolveForwardRef)
+              exports: flatten(ngModule.exports || EMPTY_ARRAY)
+                           .map(resolveForwardRef)
                            .map(expandModuleWithProviders),
               emitInline: true,
               schemas: ngModule.schemas ? flatten(ngModule.schemas) : null,
@@ -156,12 +158,12 @@ function verifySemanticsOfNgModuleDef(moduleType: NgModuleType): void {
   const errors: string[] = [];
   const declarations = maybeUnwrapFn(ngModuleDef.declarations);
   const imports = maybeUnwrapFn(ngModuleDef.imports);
-  flatten(imports, unwrapModuleWithProvidersImports).forEach(verifySemanticsOfNgModuleDef);
+  flatten(imports).map(unwrapModuleWithProvidersImports).forEach(verifySemanticsOfNgModuleDef);
   const exports = maybeUnwrapFn(ngModuleDef.exports);
   declarations.forEach(verifyDeclarationsHaveDefinitions);
   const combinedDeclarations: Type<any>[] = [
     ...declarations.map(resolveForwardRef),  //
-    ...flatten(imports.map(computeCombinedExports), resolveForwardRef),
+    ...flatten(imports.map(computeCombinedExports)).map(resolveForwardRef),
   ];
   exports.forEach(verifyExportsAreDeclaredOrReExported);
   declarations.forEach(verifyDeclarationIsUnique);
@@ -170,7 +172,8 @@ function verifySemanticsOfNgModuleDef(moduleType: NgModuleType): void {
   const ngModule = getAnnotation<NgModule>(moduleType, 'NgModule');
   if (ngModule) {
     ngModule.imports &&
-        flatten(ngModule.imports, unwrapModuleWithProvidersImports)
+        flatten(ngModule.imports)
+            .map(unwrapModuleWithProvidersImports)
             .forEach(verifySemanticsOfNgModuleDef);
     ngModule.bootstrap && ngModule.bootstrap.forEach(verifyCorrectBootstrapType);
     ngModule.bootstrap && ngModule.bootstrap.forEach(verifyComponentIsPartOfNgModule);
diff --git a/packages/core/src/util/array_utils.ts b/packages/core/src/util/array_utils.ts
index aac3dba3d8..ab846bbf7b 100644
--- a/packages/core/src/util/array_utils.ts
+++ b/packages/core/src/util/array_utils.ts
@@ -19,24 +19,23 @@ export function addAllToArray(items: any[], arr: any[]) {
 }
 
 /**
- * Flattens an array in non-recursive way. Input arrays are not modified.
+ * Flattens an array.
  */
-export function flatten(list: any[], mapFn?: (value: any) => any): any[] {
-  const result: any[] = [];
-  let i = 0;
-  while (i < list.length) {
-    const item = list[i];
+export function flatten(list: any[], dst?: any[]): any[] {
+  if (dst === undefined) dst = list;
+  for (let i = 0; i < list.length; i++) {
+    let item = list[i];
     if (Array.isArray(item)) {
-      if (item.length > 0) {
-        list = item.concat(list.slice(i + 1));
-        i = 0;
-      } else {
-        i++;
+      // we need to inline it.
+      if (dst === list) {
+        // Our assumption that the list was already flat was wrong and
+        // we need to clone flat since we need to write to it.
+        dst = list.slice(0, i);
       }
-    } else {
-      result.push(mapFn ? mapFn(item) : item);
-      i++;
+      flatten(item, dst);
+    } else if (dst !== list) {
+      dst.push(item);
     }
   }
-  return result;
-}
+  return dst;
+}
\ No newline at end of file