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

runtime/bin/vmservice/client/deployed/web/packages/browser/dart.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.
 
 (function() {
 // Bootstrap support for Dart scripts on the page as this script.
 if (navigator.userAgent.indexOf('(Dart)') === -1) {
   // TODO:
   // - Support in-browser compilation.
   // - Handle inline Dart scripts.
@@ skipping 12 matching lines @@
         var parent = scripts[i].parentNode;
         // TODO(vsm): Find a solution for issue 8455 that works with more
         // than one script.
         document.currentScript = script;
         parent.replaceChild(script, scripts[i]);
       }
     }
   }
 }
 })();