Implement proper scoping for "this" in arrow functions

src/scopeinfo.cc

Index: src/scopeinfo.cc
diff --git a/src/scopeinfo.cc b/src/scopeinfo.cc
index 10a493f5d6a13add7be2c55669aaf18b7d93bc70..333ec9ac03ba5c24abaa7b9c8a55cd7d64230c1a 100644
--- a/src/scopeinfo.cc
+++ b/src/scopeinfo.cc
@@ -279,7 +279,9 @@ bool ScopeInfo::LocalIsSynthetic(int var) {
   // with user declarations, the current temporaries like .generator_object and
   // .result start with a dot, so we can use that as a flag. It's a hack!
   Handle<String> name(LocalName(var));
-  return name->length() > 0 && name->Get(0) == '.';
+  return (name->length() > 0 && name->Get(0) == '.') ||
+         (name->length() == 4 && name->Get(0) == 't' && name->Get(1) == 'h' &&
+          name->Get(2) == 'i' && name->Get(3) == 's');
 }