Provide a Logger wrapper for write

[srawlins]

The Logger class is a great abstraction over dart:io stdout and stderr, but if I want to log some text without a newline, I cannot use Logger.

This means that I cannot catch such writes when mocking Logger with a fake Logger in tests.

Logger itself uses stdout.write in its indeterminate progress indicator (grep for stdout.write).

I think Logger should provide other commonly used methods from stdout, an IOSink, like write(), writeCharCode(), and writeln().