Re-land "Search for main in the exported names of the main library, not in the library itself."

tests/compiler/dart2js/mock_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.
 
 library mock_compiler;
 
 import "package:expect/expect.dart";
 import 'dart:async';
 import 'dart:collection';
 
@@ skipping 449 matching lines @@
     compiler.withCurrentElement(element, () {
       target.addToScope(element, compiler);
     });
   }
 }
 
 LibraryElement mockLibrary(Compiler compiler, String source) {
   Uri uri = new Uri(scheme: "source");
   var library = new LibraryElementX(new Script(uri, new MockFile(source)));
   importLibrary(library, compiler.coreLibrary, compiler);
+  library.setExports(<Element>[]);
   return library;
 }
 
 class CollectingTreeElements extends TreeElementMapping {
   final Map<Node, Element> map = new LinkedHashMap<Node, Element>();
 
   CollectingTreeElements(Element currentElement) : super(currentElement);
 
   operator []=(Node node, Element element) {
     map[node] = element;
@@ skipping 22 matching lines @@
     } else {
       sourceFile = compiler.sourceFiles[uri.toString()];
     }
     if (sourceFile != null && begin != null && end != null) {
       print(sourceFile.getLocationMessage(message, begin, end, true, (x) => x));
     } else {
       print(message);
     }
   };
 }