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

src/variables.cc

Index: src/variables.cc
diff --git a/src/variables.cc b/src/variables.cc
index c0b8bd784353aa0ad349689b30ce7578de80e116..ffa82edcb7dd2af15935cf8099d7a434006ba111 100644
--- a/src/variables.cc
+++ b/src/variables.cc
@@ -50,6 +50,15 @@ Variable::Variable(Scope* scope, const AstRawString* name, VariableMode mode,
       is_used_(false),
       initialization_flag_(initialization_flag),
       maybe_assigned_(maybe_assigned_flag) {
+  if (scope && scope->is_function_body_scope()) {
+    // The function body is the proper declaration scope, however, the
+    // declaration is added to the outer scope in order to ensure allocation
+    // works correctly. Mark the scope as the outer scope here so that
+    // checking the declaration context works.
+    scope_ = scope->outer_scope();
+    DCHECK(scope_->is_function_scope());
+  }
+
   // Var declared variables never need initialization.
   DCHECK(!(mode == VAR && initialization_flag == kNeedsInitialization));
 }