message: await openEditorAtIndex1 command
  Previously there was a bug in VSCode which prevented me from doing:
  ```
  await
  vscode.commands.executeCommand('workbench.action.openEditorAtIndex1')
  ```
  The most recent release of VSCode (1.21.0) fixes the bug. See
diff: @@ -47,8 +47,7 @@ export class TabCommand extends node.CommandBase {
  switch (this._arguments.tab) {
  case Tab.Next:
  if (this._arguments.count /** not undefined or 0 */) {
  - // do not await workbench.action.openEditorAtIndex1 because it will never resolve
  - vscode.commands.executeCommand('workbench.action.openEditorAtIndex1');
  + await vscode.commands.executeCommand('workbench.action.openEditorAtIndex1');
  await this.executeCommandWithCount(
  this._arguments.count! - 1,
  'workbench.action.nextEditorInGroup'
  @@ -68,8 +67,7 @@ export class TabCommand extends node.CommandBase {
  );
  break;
  case Tab.First:
  - // do not await workbench.action.openEditorAtIndex1 because it will never resolve
  - vscode.commands.executeCommand('workbench.action.openEditorAtIndex1');
  + await vscode.commands.executeCommand('workbench.action.openEditorAtIndex1');
  break;
  case Tab.Last:
  await vscode.commands.executeCommand('workbench.action.openLastEditorInGroup');
  