Add support for configuring the base storage URL for Flutter's
artifacts. If FLUTTER_STORAGE_BASE_URL is set, use it instead of
storage.googleapis.com.

The pub server can be overridden by setting PUB_HOSTED_URL.

This reverts commit bc4a3f17.

* work around for appveyor connectivity issues

Unfortuantelly, this slows down our build :(

* review feedback

* 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 ===

Windows' built-in unzippers are painfully slow. It drives me mad!

Unzip SDK with 7-Zip: ~10s
Unzip SDK with Windows: ~90s

This should make our scripts compatible with PowerShell 2 or newer. PowerShell 2 was released in October 2009 and shipped with Windows 7 as default. (I suspect the scripts are now also compatible with PowerShell 1, but that's unconfirmed). This fixes #8606.

The PR also introduces better error handling when Flutter fail to download the Dart SDK to fix #8627.

* for Win 7, we need to import BitsTransfer before using it
* switch to PowerShell's build-in archive expander (instead of relying on .NET)

Mainly: Don't call out to PoerShell (expansive!) if Dart SDK is upto date.

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