In the packaged ZIP files for Flutter we bundle mingit on Windows to make setup for our users easier (they don't have to download and install git anymore). This change makes Flutter use the bundled mingit if it is available and if git is not installed on the system.
This change will still require that users add `flutter\bin` to their path or they'll have to execute `flutter\bin\flutter.bat` directly. A follow-up PR will add a pre-configured Flutter Console that people can start with a double-click to minimize setup even further.
Also in this PR:
* If 7Zip is not available to unpack the Dart SDK try 7Zip's standalone version.
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.
* 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 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.
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
