[es6] implement optional parameters via desugaring (with scoping)

src/runtime/runtime-debug.cc

Index: src/runtime/runtime-debug.cc
diff --git a/src/runtime/runtime-debug.cc b/src/runtime/runtime-debug.cc
index 239f2b2e38f36ab7b30a111ce223a5f362525cb7..34d47c8b1b0d1752702b31ba20db45c015156b77 100644
--- a/src/runtime/runtime-debug.cc
+++ b/src/runtime/runtime-debug.cc
@@ -1339,6 +1339,7 @@ class ScopeIterator {
         case CATCH_SCOPE:
           DCHECK(context_->IsCatchContext());
           return ScopeTypeCatch;
+        case FUNCTION_BODY_SCOPE:
         case BLOCK_SCOPE:
           DCHECK(!scope_info->HasContext() || context_->IsBlockContext());
           return ScopeTypeBlock;