Revert "Remove Expect from core library."

tests/language/static_initializer_type_error_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";
-
 int x = "milou";
 bool inCheckedMode = false;
 
 bool readXThrows() {
   try {
     var y = x;
     return false;
   } catch (e) {
     inCheckedMode = true;
     x = 5;  // Make sure we do not throw exception a second time.
     return true;
   }
 }
 
 main() {
   int numExceptions = 0;
   for (int i = 0; i < 8; i++) {
     if (readXThrows()) {
       numExceptions++;
     }
   }
   // In checked mode throw only one exception.
   Expect.equals(inCheckedMode ? 1 : 0, numExceptions); 
 }