strip 'file:' from analyzer's output so that Sublime Text can locate the...

... offending file

(Fixed on a Windows machine.)

Two things to note:

The dart analyzer returns an error string in this format:

    file:/C:/Users/Guillermo/Desktop/foo.dart:5:1: Unexpected token '}' (expected ';')
     4: 	print("HELLO $name")
     5: }
        ~
    Compilation failed with 1 problem.

* It seems the reported path should rather look like this: `file:///...` Is that an issue in the analyzer?
* S3 does not understand the `file:///` protocol.
* S3 will fail at locating the file+line+col anyway because `/C:/...` trips it up. It only understands `/C/...`.

Lastly, I'm pretty new to Dart, so perhaps I'm not making too much sense...