Remove Expect from core library.

tests/language/full_stacktrace1_test.dart

+// (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.
+
+import "package:expect/expect.dart";
+
 void func1() {
   throw new Exception("Test peanut gallery request for Full stacktrace");
 }
 void func2() {
   func1();
 }
 void func3() {
   try {
     func2();
   } on Object catch(e, s) {
@@ skipping 36 matching lines @@
   return 1;
 }
 int func6() {
   func5();
   return 1;
 }
 main() {
   var i = func6();
   Expect.equals(1, i);
 }