Remove Expect from core library.

tests/compiler/dart2js_native/native_no_such_method_exception2_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" {
 }
 
 class B extends A native "*B" {
   foo() native;
 }
 
 makeA() native;
 makeB() native;
 
@@ skipping 23 matching lines @@
   try {
     a.foo();
   } on NoSuchMethodError catch (e) {
     exception = e;
   }
   Expect.isNotNull(exception);
 
   var b = makeB();
   Expect.equals(42, b.foo());
 }