[es6] support AssignmentPattern as LHS in for-in/of loops

src/ast/ast.cc

Index: src/ast/ast.cc
diff --git a/src/ast/ast.cc b/src/ast/ast.cc
index b8ed4a287fab5121951e7adfc49f1ad2f85190d1..29a7b2d71c13e508eeb40637a2860120d3723fef 100644
--- a/src/ast/ast.cc
+++ b/src/ast/ast.cc
@@ -137,7 +137,9 @@ static void AssignVectorSlots(Expression* expr, FeedbackVectorSpec* spec,
 void ForEachStatement::AssignFeedbackVectorSlots(
     Isolate* isolate, FeedbackVectorSpec* spec,
     FeedbackVectorSlotCache* cache) {
-  AssignVectorSlots(each(), spec, &each_slot_);
+  if (each()->IsValidReferenceExpression()) {
+    AssignVectorSlots(each(), spec, &each_slot_);
+  }
 }