Remove Expect from core library.

tests/corelib/expression_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.
 
+import "package:expect/expect.dart";
+
 // Tests basic expressions. Does not attempt to validate the details of arithmetic, coercion, and
 // so forth.
 class ExpressionTest {
 
   ExpressionTest() {}
 
   int foo;
 
   static testMain() {
     var test = new ExpressionTest();
@@ skipping 92 matching lines @@
     this[0] = 0;
     Expect.equals(42, (this[0] += $0));
     Expect.equals(129, (this[0] += $1));
     Expect.equals(246, (this[0] += $2));
   }
 }
 
 main() {
   ExpressionTest.testMain();
 }