feat(@angular-devkit/build-angular): automatically include reflect metadata polyfill for JIT

The reflect metadata polyfill is only required by Angular when using JIT.  Since it is unanticipated it would be needed when using AOT, including it when using AOT only serves to unnecessarily increase application file sizes.
JIT is not recommended for production use.  However, if an application uses JIT in production and an alternate reflect metadata polyfill is preferred (core-js/es7/reflect is currently used), then TypeScript path mapping can be used to override the polyfill.  As an example, the following will allow the usage of the `reflect-metadata` package in lieu of the default:
```
    "paths": {
      "core-js/es7/reflect": ["node_modules/reflect-metadata"]
    }
```

Author: clydin

packages/angular_devkit/build_angular/src/angular-cli-files/models/jit-polyfills.js

@@ -0,0 +1,8 @@
+/**
+ * @license
+ * Copyright Google Inc. All Rights Reserved.
+ *
+ * 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 'core-js/es7/reflect';

packages/angular_devkit/build_angular/src/angular-cli-files/models/webpack-configs/common.ts

@@ -62,6 +62,13 @@ export function getCommonConfig(wco: WebpackConfigOptions) {
     entryPoints['polyfills'] = [path.resolve(root, buildOptions.polyfills)];
   }
 
+  if (!buildOptions.aot) {
+    entryPoints['polyfills'] = [
+      ...(entryPoints['polyfills'] || []),
+      path.join(__dirname, '..', 'jit-polyfills.js'),
+    ];
+  }
+
   // determine hashing format
   const hashFormat = getOutputHashFormat(buildOptions.outputHashing as any);