[es6] implement default parameters via desugaring

src/scopes.cc

Index: src/scopes.cc
diff --git a/src/scopes.cc b/src/scopes.cc
index 60c14dbdc8d752af1cb209897c97eb7d944f553e..c60c5f7d19fe333411678d561cd50834176c5190 100644
--- a/src/scopes.cc
+++ b/src/scopes.cc
@@ -77,6 +77,7 @@ Scope::Scope(Zone* zone, Scope* outer_scope, ScopeType scope_type,
       internals_(4, zone),
       temps_(4, zone),
       params_(4, zone),
+      param_positions_(4, zone),
       unresolved_(16, zone),
       decls_(4, zone),
       module_descriptor_(
@@ -100,6 +101,7 @@ Scope::Scope(Zone* zone, Scope* inner_scope, ScopeType scope_type,
       internals_(4, zone),
       temps_(4, zone),
       params_(4, zone),
+      param_positions_(4, zone),
       unresolved_(16, zone),
       decls_(4, zone),
       module_descriptor_(NULL),
@@ -126,6 +128,7 @@ Scope::Scope(Zone* zone, Scope* inner_scope,
       internals_(0, zone),
       temps_(0, zone),
       params_(0, zone),
+      param_positions_(0, zone),
       unresolved_(0, zone),
       decls_(0, zone),
       module_descriptor_(NULL),
@@ -459,7 +462,7 @@ Variable* Scope::Lookup(const AstRawString* name) {
 
 
 Variable* Scope::DeclareParameter(const AstRawString* name, VariableMode mode,
-                                  bool is_rest, bool* is_duplicate) {
+                                  bool is_rest, bool* is_duplicate, int pos) {
   DCHECK(!already_resolved());
   DCHECK(is_function_scope());
   Variable* var = variables_.Declare(this, name, mode, Variable::NORMAL,
@@ -472,6 +475,7 @@ Variable* Scope::DeclareParameter(const AstRawString* name, VariableMode mode,
   // TODO(wingo): Avoid O(n^2) check.
   *is_duplicate = IsDeclaredParameter(name);
   params_.Add(var, zone());
+  param_positions_.Add(pos, zone());
   return var;
 }
 
@@ -1426,7 +1430,11 @@ void Scope::AllocateParameter(Variable* var, int index) {
     } else {
       DCHECK(var->IsUnallocated() || var->IsParameter());
       if (var->IsUnallocated()) {
-        var->AllocateTo(Variable::PARAMETER, index);
+        if (IsLexicalVariableMode(var->mode())) {

[caitp (gmail), 2015/05/20 17:05:16]

So this approach doesn't work --- it causes the `DCHECK(stack_locals[i]->index()
== first_slot_index + i);` in scopeinfo.cc to fail, and it doesn't really fix
anything

+          AllocateStackSlot(var);
+        } else {
+          var->AllocateTo(Variable::PARAMETER, index);
+        }
       }
     }
   }