diff --git a/packages/compiler-cli/src/ngtsc/file_system/README.md b/packages/compiler-cli/src/ngtsc/file_system/README.md
index 06a29acf08..bce731dd07 100644
--- a/packages/compiler-cli/src/ngtsc/file_system/README.md
+++ b/packages/compiler-cli/src/ngtsc/file_system/README.md
@@ -3,7 +3,15 @@
 To improve cross platform support, all file access (and path manipulation)
 is now done through a well known interface (`FileSystem`).
 
-For testing a number of `MockFileSystem` implementations are supplied.
+Note that `FileSystem` extends `ReadonlyFileSystem`, which itself extends
+`PathManipulation`.
+If you are using a file-system object you should only ask for the type that supports
+all the methods that you require.
+For example, if you have a function (`foo()`) that only needs to resolve paths then
+it should only require `PathManipulation`: `foo(fs: PathManipulation)`.
+This allows the caller to avoid implementing unneeded functionality.
+
+For testing, a number of `MockFileSystem` implementations are supplied.
 These provide an in-memory file-system which emulates operating systems
 like OS/X, Unix and Windows.
 
@@ -16,6 +24,11 @@ To prevent this happening accidentally the current file system always starts out
 as an instance of `InvalidFileSystem`, which will throw an error if any of its
 methods are called.
 
+Generally it is safer to explicitly pass file-system objects to constructors or
+free-standing functions if possible. This avoids confusing bugs where the
+global file-system has not been set-up correctly before calling functions that
+expect there to be a file-system configured globally.
+
 You can set the current file-system by calling `setFileSystem()`.
 During testing you can call the helper function `initMockFileSystem(os)`
 which takes a string name of the OS to emulate, and will also monkey-patch
diff --git a/packages/compiler-cli/src/ngtsc/file_system/index.ts b/packages/compiler-cli/src/ngtsc/file_system/index.ts
index 218f99de4b..7b25db8e8e 100644
--- a/packages/compiler-cli/src/ngtsc/file_system/index.ts
+++ b/packages/compiler-cli/src/ngtsc/file_system/index.ts
@@ -9,5 +9,5 @@ export {NgtscCompilerHost} from './src/compiler_host';
 export {absoluteFrom, absoluteFromSourceFile, basename, dirname, getFileSystem, isLocalRelativePath, isRoot, isRooted, join, relative, relativeFrom, resolve, setFileSystem, toRelativeImport} from './src/helpers';
 export {LogicalFileSystem, LogicalProjectPath} from './src/logical';
 export {NodeJSFileSystem} from './src/node_js_file_system';
-export {AbsoluteFsPath, FileStats, FileSystem, PathSegment, PathString} from './src/types';
+export {AbsoluteFsPath, FileStats, FileSystem, PathManipulation, PathSegment, PathString, ReadonlyFileSystem} from './src/types';
 export {getSourceFileOrError} from './src/util';
diff --git a/packages/compiler-cli/src/ngtsc/file_system/src/types.ts b/packages/compiler-cli/src/ngtsc/file_system/src/types.ts
index bdc660e301..6720de0b45 100644
--- a/packages/compiler-cli/src/ngtsc/file_system/src/types.ts
+++ b/packages/compiler-cli/src/ngtsc/file_system/src/types.ts
@@ -29,33 +29,14 @@ export type AbsoluteFsPath = BrandedPath<'AbsoluteFsPath'>;
 export type PathSegment = BrandedPath<'PathSegment'>;
 
 /**
- * A basic interface to abstract the underlying file-system.
- *
- * This makes it easier to provide mock file-systems in unit tests,
- * but also to create clever file-systems that have features such as caching.
+ * An abstraction over the path manipulation aspects of a file-system.
  */
-export interface FileSystem {
-  exists(path: AbsoluteFsPath): boolean;
-  readFile(path: AbsoluteFsPath): string;
-  readFileBuffer(path: AbsoluteFsPath): Uint8Array;
-  writeFile(path: AbsoluteFsPath, data: string|Uint8Array, exclusive?: boolean): void;
-  removeFile(path: AbsoluteFsPath): void;
-  symlink(target: AbsoluteFsPath, path: AbsoluteFsPath): void;
-  readdir(path: AbsoluteFsPath): PathSegment[];
-  lstat(path: AbsoluteFsPath): FileStats;
-  stat(path: AbsoluteFsPath): FileStats;
-  pwd(): AbsoluteFsPath;
-  chdir(path: AbsoluteFsPath): void;
+export interface PathManipulation {
   extname(path: AbsoluteFsPath|PathSegment): string;
-  copyFile(from: AbsoluteFsPath, to: AbsoluteFsPath): void;
-  moveFile(from: AbsoluteFsPath, to: AbsoluteFsPath): void;
-  ensureDir(path: AbsoluteFsPath): void;
-  removeDeep(path: AbsoluteFsPath): void;
-  isCaseSensitive(): boolean;
   isRoot(path: AbsoluteFsPath): boolean;
   isRooted(path: string): boolean;
-  resolve(...paths: string[]): AbsoluteFsPath;
   dirname<T extends PathString>(file: T): T;
+  extname(path: AbsoluteFsPath|PathSegment): string;
   join<T extends PathString>(basePath: T, ...paths: string[]): T;
   /**
    * Compute the relative path between `from` and `to`.
@@ -66,9 +47,41 @@ export interface FileSystem {
    */
   relative<T extends PathString>(from: T, to: T): PathSegment|AbsoluteFsPath;
   basename(filePath: string, extension?: string): PathSegment;
+  normalize<T extends PathString>(path: T): T;
+  resolve(...paths: string[]): AbsoluteFsPath;
+  pwd(): AbsoluteFsPath;
+  chdir(path: AbsoluteFsPath): void;
+}
+
+/**
+ * An abstraction over the read-only aspects of a file-system.
+ */
+export interface ReadonlyFileSystem extends PathManipulation {
+  isCaseSensitive(): boolean;
+  exists(path: AbsoluteFsPath): boolean;
+  readFile(path: AbsoluteFsPath): string;
+  readFileBuffer(path: AbsoluteFsPath): Uint8Array;
+  readdir(path: AbsoluteFsPath): PathSegment[];
+  lstat(path: AbsoluteFsPath): FileStats;
+  stat(path: AbsoluteFsPath): FileStats;
   realpath(filePath: AbsoluteFsPath): AbsoluteFsPath;
   getDefaultLibLocation(): AbsoluteFsPath;
-  normalize<T extends PathString>(path: T): T;
+}
+
+/**
+ * A basic interface to abstract the underlying file-system.
+ *
+ * This makes it easier to provide mock file-systems in unit tests,
+ * but also to create clever file-systems that have features such as caching.
+ */
+export interface FileSystem extends ReadonlyFileSystem {
+  writeFile(path: AbsoluteFsPath, data: string|Uint8Array, exclusive?: boolean): void;
+  removeFile(path: AbsoluteFsPath): void;
+  symlink(target: AbsoluteFsPath, path: AbsoluteFsPath): void;
+  copyFile(from: AbsoluteFsPath, to: AbsoluteFsPath): void;
+  moveFile(from: AbsoluteFsPath, to: AbsoluteFsPath): void;
+  ensureDir(path: AbsoluteFsPath): void;
+  removeDeep(path: AbsoluteFsPath): void;
 }
 
 export type PathString = string|AbsoluteFsPath|PathSegment;