Revert "Remove Expect from core library."

tests/corelib/expando_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";
-
 class ExpandoTest {
   static Expando<int> visits;
 
   static testMain() {
     visits = new Expando<int>('visits');
     var legal = [ new Object(),
                   new List(), [1,2,3], const [1,2,3],
                   new Map(), {'x':1,'y':2}, const {'x':1,'y':2},
                   new Expando(), new Expando('horse') ];
     for (var object in legal) {
@@ skipping 57 matching lines @@
     Expect.throws(() => expando[42.87], (exception)
                   => exception is ArgumentError);
     Expect.throws(() => expando[true], (exception)
                   => exception is ArgumentError);
     Expect.throws(() => expando[false], (exception)
                   => exception is ArgumentError);
   }
 }
 
 main() => ExpandoTest.testMain();