Fix analysis.setContextRoots failed when DAS launched from Android Studio

[EricSchlichting]

This addresses #270.

Android Studio launches dart analysis server with a very limited PATH variable. Because of this just using the command flutter or dart fails since those executables are not on the limited PATH.

If we instead use Platform.resolvedExecutable it gives us a full path to the dart executable used to run the custom lint plugin.

I removed the consideration for running flutter pub get since it looks like flutter pub get runs dart pub get under the hood and it worked fine in my testing.

❯ flutter pub get --help
Get the current package's dependencies.

Usage: dart pub get
-h, --help                Print this usage information.
    --[no-]offline        Use cached packages instead of accessing the network.
-n, --dry-run             Report what dependencies would change but don't change any.
    --enforce-lockfile    Enforce pubspec.lock. Fail resolution if pubspec.lock does not satisfy pubspec.yaml
    --[no-]precompile     Build executables in immediate dependencies.
-C, --directory=<dir>     Run this in the directory <dir>.

Run "dart help" to see global options.
See https://dart.dev/tools/pub/cmd/pub-get for detailed documentation.

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[EricSchlichting]

@rrousselGit this is ready for your re-review

[rrousselGit]

Thanks for this!