[native assets] Inconsistent logging on error.

When native assets build.dart fails during `flutter build`, its output should be printed. This doesn't currently happens consistently.

macOS (xcodebulld):
- stdout is printed
- stderr is printed twice

linux (CMake / Ninja):
- stdout is printed only with verbose flag (-v)
- stderr is printed as expected

windows (CMake / MSBuild):
- neither stdout or stderr is printed without verbose flag (-v)
- with verbose flag stderr is printed twice.

Out of these windows seems worst, as the error gets swallowed by default, and it needs to be looked up in the full log, with quite a bit of text printed after the main error.