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

sdk/lib/core/regexp.dart

Index: sdk/lib/core/regexp.dart
diff --git a/sdk/lib/core/regexp.dart b/sdk/lib/core/regexp.dart
index c5b1b5b6106c1c857920ae85e681d62a36ef78d4..c652db8d275a4a16750a3f0c379a83943d450cbd 100644
--- a/sdk/lib/core/regexp.dart
+++ b/sdk/lib/core/regexp.dart
@@ -88,13 +88,13 @@ abstract class Match {
  *     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