Refactor resolution code in the vm to properly handle ambiguity errors.

tests/language/library_ambiguous_test.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.
 
 // This test should fail to load because we are importing two libraries
 // which define the same top level name foo, and we are referring to the name.
 
-library Library1NegativeTest.dart;
 import "library1.dart";  // Defines top level variable 'foo'
 import "library2.dart";  // Defines top level variable 'foo'
 
 class X
 extends baw   /// 05: compile-time error
 {}
 
 main() {
   print(foo);  /// 00: compile-time error
   print(bar());  /// 01: compile-time error
   print(baz());  /// 02: compile-time error
   print(bay());  /// 03: compile-time error
   print(main is bax);  /// 04: compile-time error
   var x = new X();  /// 05: continued
   print("No error expected if ambiguous definitions are not used.");
 }