feat(@angular-devkit/core): hosts should return null for ENOENT

Asking for stats on an existing file should return null, but it is not
the same null (can be async and as a result) as if the host does not
support stat altogether.

Author: hansl

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

@@ -59,7 +59,7 @@ export interface ReadonlyHost<StatsT extends object = {}> {
   isFile(path: Path): Observable<boolean>;
 
   // Some hosts may not support stats.
-  stat(path: Path): Observable<Stats<StatsT>> | null;
+  stat(path: Path): Observable<Stats<StatsT> | null> | null;
 }
 
 export interface Host<StatsT extends object = {}> extends ReadonlyHost<StatsT> {

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

@@ -38,7 +38,7 @@ export interface SimpleMemoryHostStats {
 }
 
 export class SimpleMemoryHost implements Host<{}> {
-  protected readonly _cache = new Map<Path, Stats<SimpleMemoryHostStats>>();
+  protected _cache = new Map<Path, Stats<SimpleMemoryHostStats>>();
   private _watchers = new Map<Path, [HostWatchOptions, Subject<HostWatchEvent>][]>();
 
   protected _newDirStats() {
@@ -257,11 +257,11 @@ export class SimpleMemoryHost implements Host<{}> {
     return maybeStats ? maybeStats.isFile() : false;
   }
 
-  protected _stat(path: Path): Stats<SimpleMemoryHostStats> {
+  protected _stat(path: Path): Stats<SimpleMemoryHostStats> | null {
     const maybeStats = this._cache.get(this._toAbsolute(path));
 
     if (!maybeStats) {
-      throw new FileDoesNotExistException(path);
+      return null;
     } else {
       return maybeStats;
     }
@@ -343,8 +343,8 @@ export class SimpleMemoryHost implements Host<{}> {
   }
 
   // Some hosts may not support stat.
-  stat(path: Path): Observable<Stats<{}>> {
-    return new Observable<Stats<{}>>(obs => {
+  stat(path: Path): Observable<Stats<{}> | null> | null {
+    return new Observable<Stats<{}> | null>(obs => {
       obs.next(this._stat(path));
       obs.complete();
     });

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

@@ -93,7 +93,7 @@ export class SyncDelegateHost<T extends object = {}> {
 
   // Some hosts may not support stat.
   stat(path: Path): Stats<T> | null {
-    const result: Observable<Stats<T>> | null = this._delegate.stat(path);
+    const result: Observable<Stats<T> | null> | null = this._delegate.stat(path);
 
     if (result) {
       return this._doSyncCall(result);

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

@@ -116,7 +116,7 @@ export class TestHost extends SimpleMemoryHost {
 
     return super._isFile(path);
   }
-  protected _stat(path: Path): Stats<SimpleMemoryHostStats> {
+  protected _stat(path: Path): Stats<SimpleMemoryHostStats> | null {
     this._records.push({ kind: 'stat', path });
 
     return super._stat(path);