Revert "Remove Expect from core library."

tests/compiler/dart2js_native/native_class_fields_test.dart

 // Copyright (c) 2011, 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";
-
 // Verify that native fields on classes are not renamed by the minifier.
 class A native "*A" {
   int myLongPropertyName;
   int getValue;
 
   int method(int z) => myLongPropertyName;
 }
 
 
 void setup() native r"""
@@ skipping 35 matching lines @@
     // Inside this 'if' the compiler knows that a2 is an A, so it is tempted
     // to access myLongPropertyName directly, using its minified name.  But
     // renaming of native properties can only work using getters and setters
     // that access the original name.
     a2.myLongPropertyName = 21;
     int gotten = a2.myLongPropertyName;
     Expect.equals(11, gotten);
   }
 }