Revert "Remove Expect from core library."

tests/language/string_supertype_checked_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";
-
 // This tests a bug in dart2js which caused the compiler to emit bad
 // type assertions for supertypes of String.
 
 class A implements Comparable {
   int value;
 
   A(this.value);
 
   int compareTo(Comparable other) {
     A o = promote(other);
     return value.compareTo(o.value);
   }
 
   A promote(var other) {
     return other;
   }
 }
 
 main() {
   var a = new A(1);
   var b = new A(2);
   Expect.equals(-1, a.compareTo(b));
 }