Revert "Remove Expect from core library."

tests/language/string_interpolate_test.dart

 // Copyright (c) 2011, 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.
 // Dart test program testing string interpolation.
 
-import "package:expect/expect.dart";
-
 
 class WhatchamaCallIt {
   WhatchamaCallIt() { }
 
   String foo() {
     // Test $this and Field name is defined in subclass.
     return "$this and $name";
   }
 }
 
@@ skipping 38 matching lines @@
 
     var t = new ThingamaBob("Gretel");
     print(t.foo());
     Expect.equals(t.foo(), "Hansel and Gretel");
   }
 }
 
 main() {
   StringInterpolateTest.testMain();
 }