Keep track of globals declarations.

def GLOBAL(self, node):
        """
        Keep track of globals declarations.
        """
        global_scope_index = 1 if self._in_doctest() else 0
        global_scope = self.scopeStack[global_scope_index]

        # Ignore 'global' statement in global scope.
        if self.scope is not global_scope:

            # One 'global' statement can bind multiple (comma-delimited) names.
            for node_name in node.names:
                node_value = Assignment(node_name, node)

                # Remove UndefinedName messages already reported for this name.
                # TODO: if the global is not used in this scope, it does not
                # become a globally defined name.  See test_unused_global.
                self.messages = [
                    m for m in self.messages if not
                    isinstance(m, messages.UndefinedName) or
                    m.message_args[0] != node_name]

                # Bind name to global scope if it doesn't exist already.
                global_scope.setdefault(node_name, node_value)

                # Bind name to non-global scopes, but as already "used".
                node_value.used = (global_scope, node)
                for scope in self.scopeStack[global_scope_index + 1:]:
                    scope[node_name] = node_value