src/vs/platform/undoRedo/common/undoRedoService.ts/UndoRedoService/getElements
class UndoRedoService:	public getElements(resource: URI): IPastFutureElements {
		const strResource = this.getUriComparisonKey(resource);
		if (this._editStacks.has(strResource)) {
			const editStack = this._editStacks.get(strResource)!;
			return editStack.getElements();
		}
		return { past: [], future: [] };
	}