Revert "Remove Expect from core library."

tests/language/string_interpolation7_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";
-
 // Dart test program testing string interpolation with toString on custom
 // classes and on null.
 
 
 class A {
   const A();
   String toString() { return "A"; }
 }
 
 class StringInterpolation7Test {
 
   static testMain() {
     A a = new A();
     Expect.equals("A + A", "$a + $a");
     a = null;
     Expect.equals("null", "$a");
   }
 }
 
 main() {
   StringInterpolation7Test.testMain();
 }