Add "mock" compilation feature to dart2js and use it to improve test coverage.

dart/frog/leg/compiler.dart

 // Copyright (c) 2012, 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.
 
 class WorkItem {
   final Element element;
   TreeElements resolutionTree;
   Function run;
   Map<int, BailoutInfo> bailouts = null;
   bool allowSpeculativeOptimization = true;
@@ skipping 452 matching lines @@
 
   Script readScript(Uri uri, [ScriptTag node]) {
     unimplemented('Compiler.readScript');
   }
 
   String get legDirectory() {
     unimplemented('Compiler.legDirectory');
   }
 
   Element findHelper(SourceString name) => jsHelperLibrary.find(name);
+
+  bool get isMockCompilation() => false;
 }
 
 class CompilerTask {
   final Compiler compiler;
   final Stopwatch watch;
 
   CompilerTask(this.compiler) : watch = new Stopwatch();
 
   String get name() => 'Unknown task';
   int get timing() => watch.elapsedInMs();
@@ skipping 42 matching lines @@
   }
 }
 
 class SourceSpan {
   final Uri uri;
   final int begin;
   final int end;
 
   const SourceSpan(this.uri, this.begin, this.end);
 }