Split directory_posix -> directory_(android|linux|macos)
This allows us to cleanly make Android-specific changes to
directory_android.cc
Copy all _linux files to _android files and edit as needed to account
for the differences between Linux and Android:
+ getcwd(0, NULL) doesn't work on Android, have to emulate
+ Android doesn't have a '/tmp' directory, have to emulate
+ Android doesn't provide mkdtemp(), have to emulate.
+ Small differences in the available system include files.
+ Use pthread_cond_timedwait_monotonic instead of
pthread_condattr_setclock
Review URL: https://chromiumcodereview.appspot.com//10826233
git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart@10613 260f80e4-7a28-3924-810f-c04153c831b5
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