Revert "Remove Expect from core library."

tests/language/temp_mangling_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";
-
 main() {
   testOne();
   testTwo();
 }
 
 testOne() {
   // Dart2JS uses t[0-9]* to declare temporaries, so a local variable
   // with that pattern can conflict with them.
   var t0 = new List();
   // By using 'is' check, we make sure 't0.length' does not become
   // generate at use site.
   Expect.isTrue(t0.length is int);
   Expect.isTrue(t0 is List);
 }
 
 testTwo() {
   var x = new List();
   var x_0 = new List();
   {
     // This used to introduce x_0.
     var x = new Set();
     Expect.equals(0, x.length);
     Expect.isTrue(x.isEmpty);
   }
   Expect.isTrue(x is List);
   Expect.isTrue(x_0 is List);
 }