Revert "Remove Expect from core library."

tests/language/invocation_mirror_invoke_on2_test.dart

 // Copyright (c) 2013, 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";
-
 class Proxy {
   final proxied;
   Proxy(this.proxied);
   noSuchMethod(mirror) => mirror.invokeOn(proxied);
 }
 
 main() {
   testList();
   testString();
   testInt();
@@ skipping 54 matching lines @@
 testDouble() {
   var number = 42.99;
   var proxy = new Proxy(number);
 
   Expect.equals(number + 87, proxy + 87);
   Expect.equals(number.toInt(), proxy.toInt());
 
   Expect.throws(() => proxy.funky(), (e) => e is NoSuchMethodError);
   Expect.throws(() => number.funky(), (e) => e is NoSuchMethodError);
 }