Add Windows support to defaultTargetPlatform (#19176)
Add Windows support to defaultTargetPlatform (#19176)

A significant amonut of code uses defaultTargetPlatform, so it's
currently impossible to run most apps on Windows. This adds a mapping
from Windows as a host to Android as a target, paralleling the
macOS->iOS and Linux->Android mappings.

This allows more use of Windows as a host platform (e.g., for testing,
as in issue #17768).
Hixie/flutter,flutter/flutter,tvolkert/flutter,tvolkert/flutter,tvolkert/flutter,Hixie/flutter,flutter/flutter,cbracken/flutter,flutter/flutter,cbracken/flutter,flutter/flutter,flutter/flutter,cbracken/flutter,Hixie/flutter,cbracken/flutter,Hixie/flutter,Hixie/flutter,cbracken/flutter,cbracken/flutter,cbracken/flutter,tvolkert/flutter,cbracken/flutter,tvolkert/flutter,tvolkert/flutter,Hixie/flutter,cbracken/flutter,Hixie/flutter,flutter/flutter,flutter/flutter,tvolkert/flutter,flutter/flutter,cbracken/flutter,tvolkert/flutter,Hixie/flutter,Hixie/flutter,tvolkert/flutter,flutter/flutter,tvolkert/flutter,Hixie/flutter,flutter/flutter