Added `containsFile(files, file)` (basically for ceylon/ceylon-js#389)
Added `containsFile(files, file)` (basically for ceylon/ceylon-js#389)

jvasileff/ceylon-common,ceylon/ceylon-common,jvasileff/ceylon-common,ceylon/ceylon-common,ceylon/ceylon-common,jvasileff/ceylon-common