fix(@angular-devkit/schematics): better error when no schematics field in package.json

cf. angular/angular-cli#10467

Author: hansl

packages/angular_devkit/schematics/tools/node-module-engine-host.ts

@@ -5,6 +5,7 @@
  * Use of this source code is governed by an MIT-style license that can be
  * found in the LICENSE file at https://angular.io/license
  */
+import { BaseException } from '@angular-devkit/core';
 import * as core from '@angular-devkit/core/node';
 import { dirname, join, resolve as resolvePath } from 'path';
 import { RuleFactory } from '../src';
@@ -22,6 +23,13 @@ import { FileSystemEngineHostBase } from './file-system-engine-host-base';
 import { readJsonFile } from './file-system-utility';
 
 
+export class NodePackageDoesNotSupportSchematics extends BaseException {
+  constructor(name: string) {
+    super(`Package ${JSON.stringify(name)} was found but does not support schematics.`);
+  }
+}
+
+
 /**
  * A simple EngineHost that uses NodeModules to resolve collections.
  */
@@ -81,6 +89,9 @@ export class NodeModulesEngineHost extends FileSystemEngineHostBase {
       }
 
       const pkgJsonSchematics = require(packageJsonPath)['schematics'];
+      if (!pkgJsonSchematics || typeof pkgJsonSchematics != 'string') {
+        throw new NodePackageDoesNotSupportSchematics(name);
+      }
       collectionPath = this._resolvePath(pkgJsonSchematics, dirname(packageJsonPath));
     }