Revert "Remove Expect from core library."

tests/language/resolution_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.
 
-import "package:expect/expect.dart";
-
 int get foo => 499;
 
 class CompileError {
   var x;
 
   // Dart2js tried to resolve factories twice. The second time during building
   // when it was not allowed to do so.
   factory CompileError() {
     return new CompileError.internal(foo);
   }
 
   CompileError.internal(this.x);
 }
 
 void main() {
   Expect.equals(499, new CompileError().x);
 }