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

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

Index: sdk/lib/_internal/compiler/implementation/lib/core_patch.dart
diff --git a/sdk/lib/_internal/compiler/implementation/lib/core_patch.dart b/sdk/lib/_internal/compiler/implementation/lib/core_patch.dart
index f22748b6aeb733117e472ecaa147dd292868866c..1277851631dd158e3077470901b90d74aedc8073 100644
--- a/sdk/lib/_internal/compiler/implementation/lib/core_patch.dart
+++ b/sdk/lib/_internal/compiler/implementation/lib/core_patch.dart
@@ -214,3 +214,12 @@ patch class Strings {
     return array;
   }
 }
+
+patch class RegExp {
+  patch factory RegExp(String pattern,
+                       {bool multiLine: false,
+                        bool ignoreCase: false})
+    => new JSSyntaxRegExp(pattern,
+                          multiLine: multiLine,
+                          ignoreCase: ignoreCase);
+}