Revert "Remove Expect from core library."

tests/compiler/dart2js_native/native_no_such_method_exception_frog_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";
-
 class A native "*A" {
   bar() => 42;
 }
 
 class B native "*B" {
   foo() => 42;
 }
 
 makeA() native;
 
 setup() native """
   function A() {}
   makeA = function() { return new A; }
 """;
 
 main() {
   setup();
   var a = makeA();
   a.bar();
   var exception;
   try {
     a.foo();
   } on NoSuchMethodError catch (e) {
     exception = e;
   }
   Expect.isNotNull(exception);
 }