Implement proper scoping for "this" in arrow functions

src/ast.h

Index: src/ast.h
diff --git a/src/ast.h b/src/ast.h
index 4903928456dd540cce2cfbdae6f17e3c058b731e..6ef9063302d4137dc6ab5e52436ff31254db069d 100644
--- a/src/ast.h
+++ b/src/ast.h
@@ -1630,7 +1630,7 @@ class VariableProxy FINAL : public Expression {
   DECLARE_NODE_TYPE(VariableProxy)
 
   bool IsValidReferenceExpression() const OVERRIDE {
-    return !is_resolved() || var()->IsValidReference();
+    return !is_this() && (!is_resolved() || var()->IsValidReference());

[wingo, 2015/02/04 10:02:13]

So it used to be that we would eagerly resolve "this" references, and we would
fall through to the second case where var()->IsValidReference() returns false. 
But now that we create unresolved "this" references we have to eagerly prevent
unresolved "this" references from being valid left-hand-sides.

Given that, it seems to me that we need to update Variable to not have an
is_valid_ref() field, as it's not needed any more -- the only cases I could see
where it mattered were for "this".  (SuperExpression is not a ThisExpression so
the comment in variables.h is outdated.)  Can you give that a try?  Then we
remove the RHS of the && here.

[aperez, 2015/02/04 21:00:42]

Acknowledged.

Also, I am going to write it as:

  return !(is_this() || is_unresolved());

Somehow my mind can parse it more easily that way.

   }
 
   bool IsArguments() const { return is_resolved() && var()->is_arguments(); }