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

runtime/bin/vmservice/client/deployed/web/packages/mutation_observer/mutation_observer.js

 /*
  * Copyright 2013 The Polymer Authors. All rights reserved.
  * Use of this source code is goverened by a BSD-style
  * license that can be found in the LICENSE file.
  */
 
 // TODO(jmesserly): polyfill does not have feature testing or the definition of
 // SideTable. The extra code is from:
 // https://github.com/Polymer/CustomElements/blob/master/src/MutationObserver.js
 // https://github.com/Polymer/CustomElements/blob/master/src/sidetable.js
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       }
 
       clearRecords();
     }
   };
 
   global.MutationObserver = MutationObserver;
 })(window);
 
 }