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

sdk/lib/core/regexp.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.
 
 /**
  * [Match] contains methods to manipulate a regular expression match.
  *
  * Iterables of [Match] objects are returned from [RegExp] matching methods.
  *
  * The following example finds all matches of a [RegExp] in a [String]
@@ skipping 70 matching lines @@
  * Use [allMatches] to look for all matches of a regular expression in
  * a string.
  *
  * The following example finds all matches of a regular expression in
  * a string.
  *
  *     RegExp exp = new RegExp(r"(\w+)");
  *     String str = "Parse my string";
  *     Iterable<Match> matches = exp.allMatches(str);
  */
-interface RegExp extends Pattern default JSSyntaxRegExp {
+abstract class RegExp implements Pattern {
   /**
    * Constructs a regular expression. The default implementation of a
    * [RegExp] sets [multiLine] and [ignoreCase] to false.
    */
-  RegExp(String pattern, {bool multiLine: false,
-                          bool ignoreCase: false});
+  external factory RegExp(String pattern, {bool multiLine: false,
+                                           bool ignoreCase: false});
 
   /**
    * Searches for the first match of the regular expression
    * in the string [str]. Returns `null` if there is no match.
    */
   Match firstMatch(String str);
 
   /**
    * Returns an iterable on the  matches of the regular
    * expression in [str].
@@ skipping 19 matching lines @@
   /**
    * Whether this regular expression matches multiple lines.
    */
   bool get multiLine;
 
   /**
    * Whether this regular expression is case insensitive.
    */
   bool get ignoreCase;
 }