function isReferenceToGlobalVariable(node) {
			const variable = globalScope.set.get(node.name);

			return (
				variable &&
				variable.defs.length === 0 &&
				variable.references.some(ref => ref.identifier === node)
			);
		}
Checks whether the given node represents a reference to a global variable that is not declared in the source code.
These identifiers will be allowed, as it is assumed that user has no control over the names of external global variables.
@param {ASTNode} node `Identifier` node to check.
@returns {boolean} `true` if the node is a reference to a global variable.