Revert "Remove Expect from core library."

tests/compiler/dart2js_extra/constant_javascript_semantics3_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";
-
 // Make sure we use JavaScript semantics when compiling compile-time constants.
 // In this case we test that the value-range analysis uses JavaScript semantics
 // too.
 
 int inscrutable(int x) {
   if (x == 0) return 0;
   return x | inscrutable(x & (x - 1));
 }
 
 foo() {
@@ skipping 13 matching lines @@
   i--;
   i--;
   i -= 8007199254740992;   // In JS semantics [i] would be -3, now.
   return a[i];
 }
 
 main() {
   Expect.throws(() => foo(),
                 (e) => e is RangeError);
 }