Remove Expect from core library.

tests/compiler/dart2js_foreign/native_call_arity2_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.
 
 // This is a similar test to NativeCallArity1FrogTest, but makes sure
 // that subclasses also get the right number of arguments.
 
+import "package:expect/expect.dart";
 import 'native_metadata.dart';
 
 @Native("*A")
 class A {
   @native int foo([x, y]);
 }
 
 @Native("*B")
 class B extends A  {
   @native int foo([x, y]);
@@ skipping 71 matching lines @@
   Expect.equals(1, b.foo(x: 10));   // 1 = x
   Expect.equals(2, b.foo(y: 20));   // 2 = x, y
   Expect.throws(() => b.foo(10, 20, 30));
 }
 
 main() {
   setup();
   testDynamicContext();
   testStaticContext();
 }