Revert "Remove Expect from core library."

tests/language/function_malformed_result_type_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.
 // Dart test for a function with a malformed result type.
 
-import "package:expect/expect.dart";
-
 class C<T, U> {}
 
 isCheckedMode() {
   try {
     var i = 1;
     String s = i;
     return false;
   } catch (e) {
     return true;
   }
@@ skipping 16 matching lines @@
     bool got_type_error = false;
     try {
       f();
     } on TypeError catch (error) {
       got_type_error = true;
     }
     // Type error expected in checked mode only.
     Expect.isTrue(got_type_error == isCheckedMode());
   }
 }