src/vs/platform/undoRedo/common/undoRedoService.ts/EditStackSnapshot/constructor
class EditStackSnapshot:	constructor(editStacks: ResourceEditStack[]) {
		this.editStacks = editStacks;
		this._versionIds = [];
		for (let i = 0, len = this.editStacks.length; i < len; i++) {
			this._versionIds[i] = this.editStacks[i].versionId;
		}
	}