logging: introduce appenders

[DartBot]

<img src="https://avatars.githubusercontent.com/u/444270?v=3" align="left" width="96" height="96"hspace="10"> Issue by seaneagan
Originally opened as dart-lang/sdk#12028

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Currently, we have Logger.onRecord, and you can do something simple like:

logger.onRecord.map((record) => formatRecord(record)).listen(print);

But there is no easy way to log to a file (issue dart-lang/pub#458), a remote system (issue #410), etc.

In Java's logback this is handled by Appenders:

http://logback.qos.ch/manual/appenders.html

Appenders in dart could implement one of the Sink interfaces, possibly StreamSink<String> ? Maybe instead call it LineSink ? LogSink ?

then could do something like:

logger.onRecord.pipe(new ConsoleLineSink());

[DartBot]

<img src="https://avatars.githubusercontent.com/u/2909286?v=3" align="left" width="48" height="48"hspace="10"> Comment by madsager

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cc @gramster.
cc @sigmundch.
Added Area-Library, Triaged labels.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/4865287?v=3" align="left" width="48" height="48"hspace="10"> Comment by lrhn

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Removed Type-Defect label.
Added Type-Enhancement, Library-Logging labels.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/4865287?v=3" align="left" width="48" height="48"hspace="10"> Comment by lrhn

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Removed Area-Library label.
Added Area-Pkg label.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/3276024?v=3" align="left" width="48" height="48"hspace="10"> Comment by anders-sandholm

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Removed Library-Logging label.
Added Pkg-Logging label.

[natebosch]

File.openWrite() gives an IOSink. To dump a Stream<String> it could be something like:

var output = File('logs').openWrite();
logStrings.forEach(output.writeln);