Revert "Remove Expect from core library."

tests/language/parameter_name_conflict_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";
-
 foo(t0) {
   var a = t0, b = baz(), c = bar();
   if (t0 == 'foo') {
     // Force a SSA swapping problem where dart2js used to use 't0' as
     // a temporary variable.
     var tmp = c;
     c = b;
     b = tmp;
   }
 
   Expect.equals('foo', a);
   Expect.equals('foo', t0);
   Expect.equals('bar', b);
   Expect.equals('baz', c);
 }
 
 bar() => 'bar';
 baz() => 'baz';
 
 main() {
   foo('foo');
 }