[es6] Re-implement rest parameters via desugaring.

src/ast-value-factory.h

Index: src/ast-value-factory.h
diff --git a/src/ast-value-factory.h b/src/ast-value-factory.h
index ca36ac8ea19995b785a4802c56e412bbe91c27cc..105a263f03966e933388015ac70ae4eff4ccf894 100644
--- a/src/ast-value-factory.h
+++ b/src/ast-value-factory.h
@@ -262,6 +262,7 @@ class AstValue : public ZoneObject {
   F(initialize_var_global, "initializeVarGlobal")                          \
   F(is_construct_call, "_IsConstructCall")                                 \
   F(is_spec_object, "_IsSpecObject")                                       \
+  F(length, "length")                                                      \
   F(let, "let")                                                            \
   F(make_reference_error, "MakeReferenceError")                            \
   F(make_syntax_error, "MakeSyntaxError")                                  \