Revert "Remove Expect from core library."

tests/language/block_scope_test.dart

 // Copyright (c) 2011, 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";
-
 foo() => 123;
 
 void testShadowingScope1() {
   var foo = foo();
   Expect.equals(123, foo);
 }
 
 void testShadowingScope2() {
   {
     var foo = foo() + 444;
@@ skipping 46 matching lines @@
 }
 
 main() {
   testShadowingScope1();
   testShadowingScope2();
   testShadowingCapture1();
   testShadowingCapture2();
   new BlockScopeTest1().testShadowingInstanceVar();
   BlockScopeTest1.testShadowingStatic();
 }