Implement ES5 erratum: global declarations shadow inherited properties.

src/parser.cc

Index: src/parser.cc
diff --git a/src/parser.cc b/src/parser.cc
index da680411a9e4534bdd66d3221dae0f3df0fa2f9d..d13da04ad66bf956db1cb8527265761d2fc7368d 100644
--- a/src/parser.cc
+++ b/src/parser.cc
@@ -2254,7 +2254,7 @@ Block* Parser::ParseVariableDeclarations(
     // Global variable declarations must be compiled in a specific
     // way. When the script containing the global variable declaration
     // is entered, the global variable must be declared, so that if it
-    // doesn't exist (not even in a prototype of the global object) it
+    // doesn't exist (on the global object itself, see ES5 errata) it
     // gets created with an initial undefined value. This is handled
     // by the declarations part of the function representing the
     // top-level global code; see Runtime::DeclareGlobalVariable. If