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