Revert "Remove Expect from core library."

tests/language/example_constructor_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.
 // Dart test for testing order of constructor invocation.
 
-import "package:expect/expect.dart";
-
 var trace = "";
 
 int rec(int i) {
   trace = trace.concat("$i ");
   return i;
 }
 
 class A {
   A(int x) : x = rec(2) {
     Expect.equals(1, x);  // Parameter x
@@ skipping 15 matching lines @@
 
 main() {
   var test = new B(rec(0), 0, 0);
   Expect.equals(0, test.a);
   Expect.equals(2, test.x);
   Expect.equals(4, test.y);
   Expect.equals(3, test.z);
   Expect.equals("0 1 2 3 4 5 6 ", trace);
 }