frontend_server verbosity level for printing

[dcharkes]

sdk/pkg/frontend_server/lib/frontend_server.dart

Lines 355 to 374 in 302de78

	_onDiagnostic(DiagnosticMessage message) {
	bool printMessage;
	switch (message.severity) {
	case Severity.error:
	case Severity.internalProblem:
	printMessage = true;
	errors.addAll(message.plainTextFormatted);
	break;
	case Severity.warning:
	case Severity.info:
	printMessage = true;
	break;
	case Severity.context:
	case Severity.ignored:
	throw 'Unexpected severity: ${message.severity}';
	}
	if (printMessage) {
	printDiagnosticMessage(message, _outputStream.writeln);
	}
	}

The frontend_server should have a verbosity level setting and use that while deciding to print things to the configured output stream.

Currently it also prints Severity.info, leading to flutter/flutter#75429.

I'll make a CL to exclude info from prints, but the cleaner solution would be to support a verbosity level in the frontend_server.

As discussed with @johnniwinther.