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

sdk/lib/_internal/compiler/implementation/dart2js.dart

Index: sdk/lib/_internal/compiler/implementation/dart2js.dart
diff --git a/sdk/lib/_internal/compiler/implementation/dart2js.dart b/sdk/lib/_internal/compiler/implementation/dart2js.dart
index f6a91d73f10f84ed15452887f99e328b6f9217f4..423a40fded1807ce9786eb9b3a0e5760b19c67cf 100644
--- a/sdk/lib/_internal/compiler/implementation/dart2js.dart
+++ b/sdk/lib/_internal/compiler/implementation/dart2js.dart
@@ -35,7 +35,7 @@ class OptionHandler {
 String extractParameter(String argument) {
   // m[0] is the entire match (which will be equal to argument). m[1]
   // is something like "-o" or "--out=", and m[2] is the parameter.
-  Match m = const RegExp('^(-[a-z]|--.+=)(.*)').firstMatch(argument);
+  Match m = new RegExp('^(-[a-z]|--.+=)(.*)').firstMatch(argument);
   if (m == null) helpAndFail('Error: Unknown option "$argument".');
   return m[2];
 }