Remove Expect from core library.

tests/compiler/dart2js_extra/regress/4515_1_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 {
   a(x) => x + 1;
 }
 
 f(p) => p("A");
 
 main() {
   A a = new A();
   a.a(1);
   Expect.throws(() => print(f(a.a)));
 }