[es6] implement default parameters via desugaring

src/scopes.h

Index: src/scopes.h
diff --git a/src/scopes.h b/src/scopes.h
index 01df8f9d77e2d355a3ab8b17a8f7b85d5cd31990..fbbf35b9aa3552ffd59f25ddd689872e1a85bff5 100644
--- a/src/scopes.h
+++ b/src/scopes.h
@@ -126,7 +126,7 @@ class Scope: public ZoneObject {
   // parameters the rightmost one 'wins'.  However, the implementation
   // expects all parameters to be declared and from left to right.
   Variable* DeclareParameter(const AstRawString* name, VariableMode mode,
-                             bool is_rest, bool* is_duplicate);
+                             bool is_rest, bool* is_duplicate, int pos);
 
   // Declare a local variable in this scope. If the variable has been
   // declared before, the previously declared variable is returned.
@@ -373,6 +373,13 @@ class Scope: public ZoneObject {
     return params_[index];
   }
 
+  // TODO(caitp): This probably won't work when BindingPatterns are supported
+  // in function parameters. Need a better way.
+  int parameter_position(int index) const {
+    DCHECK(is_function_scope());
+    return param_positions_[index];
+  }
+
   // Returns the default function arity --- does not include rest parameters.
   int default_function_length() const {
     int count = params_.length();
@@ -553,6 +560,7 @@ class Scope: public ZoneObject {
   ZoneList<Variable*> temps_;
   // Parameter list in source order.
   ZoneList<Variable*> params_;
+  ZoneList<int> param_positions_;
   // Variables that must be looked up dynamically.
   DynamicScopePart* dynamics_;
   // Unresolved variables referred to from this scope.