Revert "Remove Expect from core library."

tests/language/string_interpolate2_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.
 // Dart test program testing string interpolation of expressions.
 
-import "package:expect/expect.dart";
-
 
 class StringInterpolate2Test {
 
   static var F1;
 
   static void testMain() {
   
     F1 = "1 + 5 = ${1+5}";
     
     Expect.equals("1 + 5 = 6", F1);
@@ skipping 31 matching lines @@
     // multiline string types
     s = "a ${(){ return 'b ${(){ return """
 c""";}()}'; }()} d";
     Expect.equals("a b c d", s);
   }
 }
 
 main() {
   StringInterpolate2Test.testMain();
 }