[es6] Fix scoping for default parameters in arrow functions

src/parser.cc

Index: src/parser.cc
diff --git a/src/parser.cc b/src/parser.cc
index 666b7043468d590852ea2b914d3fc175ba39718e..f0affc27985c5e8b68f7bfbd3e94755e9cb96426 100644
--- a/src/parser.cc
+++ b/src/parser.cc
@@ -14,6 +14,7 @@
 #include "src/codegen.h"
 #include "src/compiler.h"
 #include "src/messages.h"
+#include "src/parameter-initializer-rewriter.h"
 #include "src/preparser.h"
 #include "src/rewriter.h"
 #include "src/runtime/runtime.h"
@@ -4049,6 +4050,10 @@ void ParserTraits::ParseArrowFunctionFormalParameters(
     DCHECK(!assignment->is_compound());
     initializer = assignment->value();
     expr = assignment->target();
+
+    // TODO(adamk): Only call this if necessary.
+    RewriteParameterInitializerScope(parser_->stack_limit(), initializer,
+                                     parser_->scope_, parameters->scope);
   }
 
   AddFormalParameter(parameters, expr, initializer, is_rest);