Introduce global contexts to represent lexical global scope(s).

src/scopes.h

Index: src/scopes.h
diff --git a/src/scopes.h b/src/scopes.h
index e7832d354e114dd341b6e4eb4afc8feef313c1fd..6e35d05a7f5d07f3b9941b8489006fed3aa03530 100644
--- a/src/scopes.h
+++ b/src/scopes.h
@@ -160,7 +160,7 @@ class Scope: public ZoneObject {
   // global scope.  The variable was introduced (possibly from an inner
   // scope) by a reference to an unresolved variable with no intervening
   // with statements or eval calls.
-  Variable* DeclareGlobal(Handle<String> name);
+  Variable* DeclareDynamicGlobal(Handle<String> name);
 
   // Create a new unresolved variable.
   template<class Visitor>