replaced path.join with path.url.join (#1562)
replaced path.join with path.url.join (#1562)

* replaced path.join with path.url.join

* Changed p.normalize to p.url.normalize and p.dirname to p.url.dirname to be independent of host OS.

* Ran dartfmt

dart-lang/build,dart-lang/build,dart-lang/build,dart-lang/build