fix(build): declare external dep on angular/core in webpack bundle

Closes #2687

Author: christopherthielen

packages/ng2/webpack.config.js

@@ -43,6 +43,6 @@ module.exports = {
   },
 
   externals: {
-    "angular2/core": { root: 'angular2/core', commonjs2: 'angular2/core', commonjs: 'angular2/core' }
+    "angular2/core": { root: 'angular2/core', amd: 'angular2/core', commonjs2: 'angular2/core', commonjs: 'angular2/core' }
   }
 };