Remove Expect from core library.

tests/compiler/dart2js_native/native_call_arity3_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";
+
 // Test similar to NativeCallArity1FrogTest, but with default values to
 // parameters set to null. These parameters should be treated as if they
 // do not have a default value for the native methods.
 
 class A native "*A" {
   int foo(int x) native;
 }
 
 class B native "*B" {
   int foo([x = null, y, z = null]) native;
@@ skipping 56 matching lines @@
   Expect.equals(2, b.foo(null, 20));
   Expect.equals(3, b.foo(null, null, 30));
   Expect.throws(() => b.foo(10, 20, 30, 40));
 }
 
 main() {
   setup();
   testDynamicContext();
   testStaticContext();
 }