feat(@angular-devkit/core): support resetting a memory host

Author: clydin

etc/api/angular_devkit/core/src/_golden-api.d.ts

@@ -992,6 +992,7 @@ export declare class SimpleMemoryHost implements Host<{}> {
     list(path: Path): Observable<PathFragment[]>;
     read(path: Path): Observable<FileBuffer>;
     rename(from: Path, to: Path): Observable<void>;
+    reset(): void;
     stat(path: Path): Observable<Stats<{}> | null> | null;
     watch(path: Path, options?: HostWatchOptions): Observable<HostWatchEvent> | null;
     write(path: Path, content: FileBuffer): Observable<void>;

packages/angular_devkit/core/src/virtual-fs/host/memory.ts

@@ -371,4 +371,9 @@ export class SimpleMemoryHost implements Host<{}> {
   watch(path: Path, options?: HostWatchOptions): Observable<HostWatchEvent> | null {
     return this._watch(path, options);
   }
+
+  reset(): void {
+    this._cache.clear();
+    this._watchers.clear();
+  }
 }