Revert "Remove Expect from core library."

tests/compiler/dart2js_foreign/native_call_arity3_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.
 
 // 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.
 
-import "package:expect/expect.dart";
 import 'native_metadata.dart';
 
 @Native("*A")
 class A {
   @native int foo(int x);
 }
 
 @Native("*B")
 class B {
   @native int foo([x = null, y, z = null]);
@@ skipping 57 matching lines @@
   Expect.equals(2, b.foo(y: 20));
   Expect.equals(3, b.foo(z: 30));
   Expect.throws(() => b.foo(10, 20, 30, 40));
 }
 
 main() {
   setup();
   testDynamicContext();
   testStaticContext();
 }