[dart_runner] Allow exit codes from main()

The exit code from fuchsia.exit(value) was being overidden.

Also, initialize has_set_return_code in DartState.

TEST: Wrote a test that runs goodbye_dart in two modes and checks the
results. Tested the test by changing the return code :)

US-510 #done

Change-Id: I4af2f7927dc0a8e0c784c4eb1377367f3156f498
1 file changed