Remove Expect from core library.

tests/compiler/dart2js_extra/constant_javascript_semantics2_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) => x == 0 ? 0 : x | inscrutable(x & (x - 1));
 
 foo() {
   var a = const [1, 2];
   var i = 0x100000000;
   if (inscrutable(i) == 0) {
     i = 0x100000001;
   }
   i = 0xFFFFFFFFF & i;  // In JS semantics [:i:] will be truncated to 32 bits.
   i = 0x100000001 - i;
   return a[i];
 }
 
 main() {
   Expect.throws(() => foo(),
                 (e) => e is RangeError);
 }