Revert "Remove Expect from core library."

tests/language/metadata_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.
 
 // Test ensuring that compiler can parse metadata. Need to add negative
 // test cases with illegal metadata annotations.
 
 library metadata_test.dart;
-import "package:expect/expect.dart";
 import "metadata_lib.dart" as Meta;
 
 class Tag {
   final String annotation;
   const Tag(this.annotation);
 }
 
 const meta1 = 1;
 const meta2 = const Tag("meta2");
 
@@ skipping 19 matching lines @@
 @meta1 main() {
   @meta1 var a = new A();
   Expect.equals(0, a.foo(false, x: 0));
 
   for (@Tag("loopvar") int i = 0; i < 10; i++) {
     // Do something.
   }
 
   @meta1 var s = r'This is a raw \\ string.';
 }