Remove asyncGenerator from babel-helpers

[razh]

asyncGenerator currently cannot be removed by rollup's tree-shaking
from rollup so it has to be done manually.

See rollup/rollup-plugin-babel#100 (comment)

Examples of this same config option:

• redux
• redux-saga

[houndci-bot]

Some files could not be reviewed due to errors:

Oops! Something went wrong! :(
ESLint: 4.9.0.
ESLint couldn't find the plugin "eslint-plugin-vue". This can happen for a couple different reasons:
1. If ESLint is installed globally, then make sure eslint-plugin-vue is also installed globally. A globally-installed ESLint cannot find a locally-installed plugin.
2. If ESLint is installed locally, then it's likely that the plugin isn't installed correctly. Try reinstalling by running the following:
    npm i eslint-plugin-vue@latest --save-dev
If you still can't figure out the problem, please stop by https://gitter.im/eslint/eslint to chat with the team.

[razh]

This is no longer necessary. rollup 0.51 will remove this by default: https://github.com/rollup/rollup/blob/master/CHANGELOG.md#0510

[akxcv]

Thanks! I'll take a look at this when I have the time.