Introduce with scope and rework variable resolution.

src/variables.h

Index: src/variables.h
diff --git a/src/variables.h b/src/variables.h
index 56c8dabd378e033647a5563d63721e1224363047..0cd1d9e6ae0c7cc8674907c53113f5852a89e11f 100644
--- a/src/variables.h
+++ b/src/variables.h
@@ -182,6 +182,10 @@ class Variable: public ZoneObject {
   Location location_;
   int index_;
 
+  // If this field is set, this variable references the stored locally bound
+  // variable, but it might be shadowed by variable bindings introduced by
+  // non-strict 'eval' calls between the reference scope (inclusive) and the
+  // binding scope (exclusive).
   Variable* local_if_not_shadowed_;
 
   // Valid as a LHS? (const and this are not valid LHS, for example)