Revert "Remove Expect from core library."

tests/language/const_locals_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";
-
 // Test of compile time constant local variables.
 
 const N = 8;
 
 class ConstFoo {
   final x;
   const ConstFoo(this.x);
 }
 
 main() {
   const MIN = 2 - 1;
   const MAX = N * 2;
   const MASK = (1 << (MAX - MIN + 1)) - 1;  // 65535.
   Expect.equals(1, MIN);
   Expect.equals(16, MAX);
   Expect.equals(65535, MASK);
   const s = 'MIN = $MIN  MAX = $MAX  MASK = $MASK';
   Expect.identical(s, 'MIN = $MIN  MAX = $MAX  MASK = $MASK');
   Expect.equals("MIN = 1  MAX = 16  MASK = 65535", s);
   var cf1 = const ConstFoo(MASK);
   var cf2 = const ConstFoo(s);
   var cf3 = const ConstFoo('MIN = $MIN  MAX = $MAX  MASK = $MASK');
   Expect.identical(cf2, cf3);
   Expect.isFalse(identical(cf2, cf1));
 }