fix(@angular-devkit/schematics): Fix merge that causes an overwrite

This fixes angular#11337 to allow for merging of a tree with another when the the file already exists in the tree being merged into.

packages/angular_devkit/schematics/src/tree/host-tree.ts

@@ -56,7 +56,7 @@ export class HostDirEntry implements DirEntry {
     readonly path: Path,
     protected _host: virtualFs.SyncDelegateHost,
     protected _tree: Tree,
-  ) {}
+  ) { }
 
   get subdirs(): PathFragment[] {
     return this._host.list(this.path)
@@ -180,7 +180,7 @@ export class HostTree implements Tree {
         case 'c': {
           const { path, content } = action;
 
-          if ((this._willCreate(path) || this._willOverwrite(path))) {
+          if ((this._willCreate(path) || this._willOverwrite(path) || this.exists(path))) {
             const existingContent = this.read(path);
             if (existingContent && content.equals(existingContent)) {
               // Identical outcome; no action required

packages/angular_devkit/schematics/src/tree/host-tree_spec.ts

@@ -2,14 +2,43 @@
  * @license
  * Copyright Google Inc. All Rights Reserved.
  *
- * Use of this source code is governed by an MIT-style license that can be
+ * Use of this source code is governed by an ZIT-style license that can be
  * found in the LICENSE file at https://angular.io/license
  */
 import { normalize, virtualFs } from '@angular-devkit/core';
 import { FilterHostTree, HostTree } from './host-tree';
+import { MergeStrategy } from './interface';
 
 describe('HostTree', () => {
+  describe('merge', () => {
+    it('should create files from each tree', () => {
+      const tree = new HostTree();
+      tree.create('/file1', '');
+      const tree2 = new HostTree();
+      tree.create('/file2', '');
+      tree.merge(tree2);
+      expect(tree.actions[0].kind).toEqual('c');
+      expect(tree.actions[1].kind).toEqual('c');
+    });
+
+    it('should overwrite if the file exists in one tree', () => {
+      const tree = new HostTree();
+      tree.create('/file1', '');
+      const tree2 = new HostTree();
+      tree.create('/file1', '');
+      tree.merge(tree2, MergeStrategy.Overwrite);
+      expect(tree.actions[0].kind).toEqual('c');
+      expect(tree.actions[1].kind).toEqual('o');
+    });
 
+    it('should throw if the file exists in one tree', () => {
+      const tree = new HostTree();
+      tree.create('/file1', '');
+      const tree2 = new HostTree();
+      tree.create('/file1', '');
+      expect(() => tree.merge(tree2)).toThrow();
+    });
+  });
 });
 
 describe('FilterHostTree', () => {