Flag Sink objects that are never closed

[filiph]

class A {
  IOSink _sink;
  void init(filename) {
    _sink = new File(filename).openWrite();
  }
}

Here's another linter hint that's standard-lib-specific (something like C linter hints complaining about unsafe use of memcpy, malloc, etc.). Whenever there is a Sink object that is opened in scope but never called close() upon in that same scope, that's a pretty strong indication the developer forgot.

We could easily identify other similar classes that have to be closed (or unsubscribed from) when no longer in use.

[alexeieleusis]

This should be fixed now dart-lang/linter#241
small related variant in dart-lang/linter#242 landing soon

[pq]

Implemented in close_sinks. Thanks @alexeieleusis!