Return correct exit code from FormatCommand when formatting stdin.

[munificent]

It reads from stdin asynchronously. Before this fix, the FormatCommand
did not wait for that to complete before returning the exit code, so it
would return before stdin had been formatted and any parse error
detected.

Fix #1035.

[mit-mit]

Formatted 47 files (1 changed) in 0.80 seconds.
Error: Process completed with exit code 1.

Oh the irony when a dart_style PR fails the formatting check; you should use for formatter Bob! 😂