Remove Expect from core library.

runtime/tests/vm/dart/optimized_stacktrace_test.dart

 // Copyright (c) 2013, 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 correct source positions in stack trace with optimized functions.
+import "package:expect/expect.dart";
 
 // (1) Test normal exception. 
 foo(x) => bar(x);
 
 bar(x) {
   if (x == null) throw 42;  // throw at position 11:18
   return x + 1;
 }
 
 test1() {
@@ skipping 53 matching lines @@
     Expect.notEquals(-1, s.indexOf("maximus"));
     Expect.notEquals(-1, s.indexOf("moritz"));
     Expect.equals(-1, s.indexOf("-1:-1"));
   }
 }
 
 main() {
   test1();
   test2();
 }