emitter generates relative import URLs with backslashes on Windows

[srawlins]

I don't have a repro for this; this comes from https://github.com/dart-lang/mockito/issues/591 where @adbonnin has a repro.

Backslashes should not be used in relative import URLs.

The line of code in question is probably https://github.com/dart-lang/code_builder/blob/master/lib/src/emitter.dart#L336