Remove Expect from core library.

tests/language/string_interpolation_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";
+
 // Tests for string interpolation
 class StringInterpolationTest {
 
   StringInterpolationTest() {}
 
   static void m() {}
 
   static const int i = 1;
   static const String a = "<hi>";
 
@@ skipping 57 matching lines @@
   }
 
   String embedMultipleFields() {
     return "j = ${j}; k = ${k}";
   }
 }
 
 main() {
   StringInterpolationTest.testMain(false);
 }