Revert "Remove Expect from core library."

tests/language/propagated_argument_type_check_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";
-
 main() {
   Expect.equals("str", foo("str"));
 }
 
 foo(y) {
   var x = 3;
   for (int i = 0; i < 2; i++) {
     // Make sure that we don't think that the type of x is necessarily
     // a number and optimize the x + y expression based on that. The
     // value of x changes later...
     if (i == 1) return bar(x + y);
     x = new A();
   }
 }
 
 bar(t) => t;
 
 class A {
   A() {}
   operator +(x) => x;
 }