Revert "Remove Expect from core library."

tests/language/type_propagation_phi_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.
 // Check that phi type computation in the Dart2Js compiler does the
 // correct thing.
 
-import "package:expect/expect.dart";
-
 bar() => 490;
 bar2() => 0;
 
 foo(b) {
   var x = bar();
   var x2 = x;
   if (b) x2 = bar2();
   var x3 = 9 + x;  // Guarantees that x is a number. Dart2js propagated the
                    // type information back to the phi (for x2).
   return x2 + x3;
 }
 
 main() {
   Expect.equals(499, foo(true));
 }