Implement proper scoping for "this" in arrow functions

src/ast.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_AST_H_
 #define V8_AST_H_
 
 #include "src/v8.h"
 
 #include "src/assembler.h"
@@ skipping 1612 matching lines @@
   Handle<FixedArray> constant_elements_;
   ZoneList<Expression*>* values_;
 };
 
 
 class VariableProxy FINAL : public Expression {
  public:
   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the comment in variables.h is outdated.)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(); }
 
   Handle<String> name() const { return raw_name()->string(); }
   const AstRawString* raw_name() const {
     return is_resolved() ? var_->raw_name() : raw_name_;
   }
 
   Variable* var() const {
@@ skipping 1893 matching lines @@
 
  private:
   Zone* zone_;
   AstValueFactory* ast_value_factory_;
 };
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_AST_H_