Revert "Remove Expect from core library."

tests/language/interceptor_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";
-
 // Test that interceptors (that is, methods in classes implemented as
 // JavaScript primitives) in dart2js work.
 
 class A {
   codeUnitAt(a) => a;
 }
 
 main() {
   var res = [[], 1, 'foo', new A()];
   Expect.throws(() => res[0].codeUnitAt(1));
   Expect.throws(() => (res[0].codeUnitAt)(1));
 
   Expect.throws(() => res[1].codeUnitAt(1));
   Expect.throws(() => (res[1].codeUnitAt)(1));
 
   Expect.equals(111, res[2].codeUnitAt(1));
   Expect.equals(111, (res[2].codeUnitAt)(1));
   Expect.throws(() => res[2].codeUnitAt(1, 4));
   Expect.throws(() => res[2].codeUnitAt());
   Expect.throws(() => (res[2].codeUnitAt)(1, 4));
   Expect.throws(() => (res[2].codeUnitAt)());
 
   Expect.equals(1, res[3].codeUnitAt(1));
   Expect.equals(1, (res[3].codeUnitAt)(1));
   Expect.throws(() => res[3].codeUnitAt(1, 4));
   Expect.throws(() => res[3].codeUnitAt());
   Expect.throws(() => (res[3].codeUnitAt)(1, 4));
   Expect.throws(() => (res[3].codeUnitAt)());
 }