Remove Expect from core library.

tests/language/const_syntax_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";
+
 main() {
   const f0 = 42;
   const f1; /// 01: compile-time error
   const int f2 = 87;
   const int f3; /// 02: compile-time error
   Expect.equals(42, f0);
   Expect.equals(87, f2);
 
   Expect.equals(42, F0);
   Expect.equals(null, F1); /// 03: compile-time error
@@ skipping 51 matching lines @@
       : x = C0.X /// 09: continued
   ;
   final x = null;
 }
 
 // Check that sub-expressions of binary + are numeric.
 const B0 = 42;
 const B1 = "Hello";
 const B2 = "$B1 $B0";
 const B3 = B0 + B1; /// 10: continued