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

utils/apidoc/html_diff.dart

Index: utils/apidoc/html_diff.dart
diff --git a/utils/apidoc/html_diff.dart b/utils/apidoc/html_diff.dart
index 130666d2b9ae9e76aadf69c60c7fcd2856bc5c62..49685b28af37f52d4169b4f3983e2ac3fdc47097 100644
--- a/utils/apidoc/html_diff.dart
+++ b/utils/apidoc/html_diff.dart
@@ -8,7 +8,6 @@
  */
 library html_diff;
 
-import 'dart:coreimpl';
 import 'dart:io';
 
 // TODO(rnystrom): Use "package:" URL (#4968).
@@ -228,7 +227,7 @@ class HtmlDiff {
    */
   Map<String, String> _getTags(String comment) {
     if (comment == null) return const <String, String>{};
-    final re = const RegExp("@([a-zA-Z]+) ([^;]+)(?:;|\$)");
+    final re = new RegExp("@([a-zA-Z]+) ([^;]+)(?:;|\$)");
     final tags = <String, String>{};
     for (var m in re.allMatches(comment.trim())) {
       tags[m[1]] = m[2];