Switch to using pub build instead of custom build_.dart script

runtime/bin/vmservice/client/deployed/web/packages/browser/interop.js

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-library canonicalization.a;
-
-import 'package:polymer/polymer.dart';
-import 'package:canonicalization/c.dart';
-import 'd.dart';
-
-int a = 0;
-@initMethod initA() {
-  a++;
-  c++;
-  d++;
+// Type for remote proxies to Dart objects with dart2js.
+// WARNING: do not call this constructor or rely on it being
+// in the global namespace, as it may be removed.
+function DartObject(o) {
+  this.o = o;
 }