Revert "Remove Expect from core library."

tests/language/value_range2_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";
-
 int inscrutable(int x) => x == 0 ? 0 : x | inscrutable(x & (x - 1));
 
 foo() {
   int x = 0;
   if (inscrutable(0) == 0) x = -2;  // x is now in range [-2 .. 0].
   int y = 2;
   if (inscrutable(0) == 0) y = 4;  // y is now in range [2 .. 4].
   int i = y - x;  // i should be in range [2 .. 6].
   i -= 4;  // i should be in range [-2 .. 2]. Actual value: 2.
   var a = const [1];
   return a[i];
 }
 
 main() {
   Expect.throws(() => foo(),
                 (e) => e is RangeError);
 }