Remove Expect from core library.

tests/compiler/dart2js_native/native_missing_method2_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" {
 }
 
 makeA() native;
 
 void setup() native """
 function A() {};
 A.prototype.foo = function() { return  42; }
 makeA = function() { return new A; }
 """;
@@ skipping 28 matching lines @@
                 (e) => e is NoSuchMethodError);
   Expect.throws(() => a.foo = 4,
                 (e) => e is NoSuchMethodError);
 }
 
 main() {
   setup();
   typedContext();
   untypedContext();
 }