Revert "Remove Expect from core library."

tests/language/compile_time_constant_a_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";
-
 const m1 = const { 'a': 400 + 99 };
 const m2 = const { 'a': 499, 'b': 42 };
 const m3 = const { 'm1': m1, 'm2': m2 };
 const m4 = const { 'z': 9, 'a': 8, 'm': 7 };
 const m5 = const { '': 499 };
 const m6 = const { 'a': 499 };
 const m7 = const {};
 
 bool isUnsupportedError(o) => o is UnsupportedError;
 
@@ skipping 98 matching lines @@
   Expect.throws(() => m7.remove('a'), isUnsupportedError);
   Expect.throws(() => m7.remove('b'), isUnsupportedError);
   Expect.throws(() => m7.clear(), isUnsupportedError);
   Expect.throws(() => m7['b'] = 42, isUnsupportedError);
   Expect.throws(() => m7['a'] = 499, isUnsupportedError);
   Expect.throws(() => m7.putIfAbsent('a', () => 499),
                 isUnsupportedError);
   Expect.throws(() => m7.putIfAbsent('z', () => 499),
                 isUnsupportedError);
 }