* Support 'be' channel so you can download dart-sdk from the tip.
This is triggered when you put 'hash/<build hash>' into dart-sdk.version file. For example, 'hash/c0617d20158955d99d6447036237fe2639ba088c'
* Add README.md
* Fix grammar, spacing, 80 chars, nicer ===
This brings the Windows script up to par with the Linux/Mac script.
`flutter doctor` works.
There are two scripts (`bin/flutter` and `bin/internal/update_dart-sdk`), which are platform dependent (bash script on Linux/Max, PowerShell script on Windows) and we need to keep an eye on making sure that their logic stays in sync. Both scripts are rather simple and I am not expecting many changes to them. I also made sure that both versions follow the same structure to make it easier to keep them consistent.
Required for https://github.com/flutter/flutter/issues/138
