Allow for the creation of multiple Flutter SDKs

[pq]

Before long we'll want to allow users to move between multiple SDKs (possibly allowing for per-project configuration a la the conversation in #54). To facilitate this, we should support SDK configuration like the Java tooling does.

For example, where for Java we might see:

we could imagine something similar for Flutter like:

[pq]

Relatedly, we'll want to settle on a good version numbering scheme. (The 0.0.1 in flutter\VERSION isn't distinguishing enough.)

@sethladd : thoughts?

Ideally we'd have something like 0.0.1-SOMETHING where SOMETHING is meaningful and comparable like a date (vs. a SHA).

[sethladd]

@abarth has some thoughts about versioning flutter, and it's related to how flutter sdk plays with pub. Thanks for queuing this up. Is this something you need a decision on now?

[pq]

Nope. Not super pressing but it'd be handy.

Is there another issue out there that I can track?

[abarth]

This is a complex topic. Happy to discuss but you might want to wait until I'm back in the office.

[pq]

Sounds good!

[pq]

This issue is well understood. Closing in favor of opening more targeted tracking issues down the line.

[alexander-doroshko]

It depends on Dart SDK per module support that is not yet done. It's in my list for 2017.1. Tracked as https://youtrack.jetbrains.com/issue/WEB-20870