Revert "Remove Expect from core library."

tests/compiler/dart2js/unparser2_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";
 import "../../../sdk/lib/_internal/compiler/implementation/scanner/scannerlib.dart";
 import "../../../sdk/lib/_internal/compiler/implementation/tree/tree.dart";
 
 import "../../../sdk/lib/_internal/compiler/implementation/dart2jslib.dart"
     show DiagnosticListener,
          Script;
 
 import "../../../sdk/lib/_internal/compiler/implementation/elements/elements.dart"
     show CompilationUnitElement,
          LibraryElement;
@@ skipping 68 matching lines @@
   }
   void internalErrorOnElement(Element element, String message) {
     throw message;
   }
   void internalError(String message,
                      {Node node, Token token, dynamic instruction,
                       Element element}) {
     throw message;
   }
 }