src/vs/platform/contextkey/browser/contextKeyService.ts/ScopedContextKeyService/constructor
class ScopedContextKeyService:	constructor(parent: AbstractContextKeyService, domNode: IContextKeyServiceTarget) {
		super(parent.createChildContext());
		this._parent = parent;
		this._updateParentChangeListener();

		this._domNode = domNode;
		if (this._domNode.hasAttribute(KEYBINDING_CONTEXT_ATTR)) {
			let extraInfo = '';
			if ((this._domNode as HTMLElement).classList) {
				extraInfo = Array.from((this._domNode as HTMLElement).classList.values()).join(', ');
			}

			console.error(`Element already has context attribute${extraInfo ? ': ' + extraInfo : ''}`);
		}
		this._domNode.setAttribute(KEYBINDING_CONTEXT_ATTR, String(this._myContextId));
	}