Revert "Remove Expect from core library."

tests/language/dangling_else_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.
 // Tests dangling else. The VM should not have any problems, but dart2js or
 // dart2dart could get this wrong.
 
-import "package:expect/expect.dart";
-
 nestedIf1(notTrue) {
   if (notTrue) return 'bad input';
   if (notTrue) {
     if (notTrue) {
       return 'bad';
     }
   } else {
     return 'good';
   }
   return 'bug';
@@ skipping 56 matching lines @@
   return 'bug';
 }
 
 main() {
   Expect.equals('good', nestedIf1(false));
   Expect.equals('good', nestedIf2(false));
   Expect.equals('good', nestedWhile(false));
   Expect.equals('good', nestedFor(false));
   Expect.equals('good', nestedLabeledStatement(false));
 }