Revert "Remove Expect from core library."

tests/language/value_range_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() {
   var x = 0x102;
   if (inscrutable(x) == 0) x = 0x0;
   if (inscrutable(10) == 10) x = 0x10;  // x is in range [0 .. 0x102].
   x = x & 0xFF;  // x should be in range [0 .. 0xFF]. Actual value: 0x10.
   var a = const [1, 2, 3];
   return a[x];
 }
 
 main() {
   Expect.throws(() => foo(),
                 (e) => e is RangeError);
 }