Force lf-line endings for so that source offsets match across platforms. (#6408)

.gitattributes

-## This page intentionally left blank. ##
-#
-# Workaround for VS2013 automatically creating .gitattributes files with
-# default settings that we don't want.
-# See also:
-# http://connect.microsoft.com/VisualStudio/feedback/details/804948/inappropriately-creates-gitattributes-file
-# http://crbug.com/342064
+# Dart sources have to be normalized so that source offsets in kernel files
+# with source embedded in them match across all platforms.
+*.dart text eol=lf
\ No newline at end of file