Move JSSyntaxRegExp to core as a private member. This removes the last refrences to dart:coreimpl.

sdk/lib/coreimpl/regexp.dart

Index: sdk/lib/coreimpl/regexp.dart
diff --git a/sdk/lib/coreimpl/regexp.dart b/sdk/lib/coreimpl/regexp.dart
deleted file mode 100644
index b7076311f31afeffbca66a6570cf538bb68d9552..0000000000000000000000000000000000000000
--- a/sdk/lib/coreimpl/regexp.dart
+++ /dev/null
@@ -1,16 +0,0 @@
-// 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.
-
-class JSSyntaxRegExp implements RegExp {
-  external const JSSyntaxRegExp(String pattern,
-                                {bool multiLine: false,
-                                 bool ignoreCase: false});
-  external Match firstMatch(String str);
-  external Iterable<Match> allMatches(String str);
-  external bool hasMatch(String str);
-  external String stringMatch(String str);
-  external String get pattern;
-  external bool get multiLine;
-  external bool get ignoreCase;
-}