Remove fast-async from the web preset

[edmorley]

The web preset currently adds fast-async to the babel plugins list by default. The plugin provides a faster alternative to transform-async-to-generator.

However:

• the web preset's default target browsers all now support async-functions natively (hard to believe, but true - caniuse !!) and so babel-preset-env doesn't even enable transform-async-to-generator for them (see here)
• fast-aysnc is soon going to be merged into the core babel packages (Implement transform-async-to-promises babel/babel#7076) - so I'm guessing will be included in the babel 7 release, which we'll be updating to before shipping Neutrino 9 final. And so at that point we definitely won't need to include fast-async, regardless.

...as such I think we should be fine to remove fast-async in Neutrino 9, thereby simplifying the babel config in the web preset, and reducing the number of dependencies.

Note something similar could be done for the node preset, however it would need to wait until node 6 support was dropped - node.green.

[eliperelman]

This is pretty amazing. For v9, I propose that we exclude fast-async for the Node.js preset as well and switch the matrix of support Node.js v8, since that will be current LTS at the time of release.