Revert "Remove Expect from core library."

tests/compiler/dart2js_extra/break_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";
-
 break1(int x, int y, int ew, int ez) {
   int w = 1;
   int z = 0;
   bk1: if (x == 2) {
     z = 1;
     if (y == 3) {
       w = 2;
       break bk1;
     } else {
       w = 3;
@@ skipping 126 matching lines @@
   Expect.isTrue(obscureBreaks(3), "3");
   Expect.isTrue(obscureBreaks(4), "4");
   Expect.isFalse(obscureBreaks(5), "5");
   Expect.equals(3, ifBreaks(1, 4));
   Expect.equals(4, ifBreaks(1, 2));
   Expect.equals(5, ifBreaks(1, 3));
   Expect.equals(6, ifBreaks(2, 4));
   Expect.equals(7, ifBreaks(2, 2));
   Expect.equals(8, ifBreaks(2, 3));
 }