Make Directory::Current() work with unenhanced POSIX getcwd

Make Directory::Current() work with unenhanced POSIX getcwd

POSIX says getcwd needs to be passed a pre-allocated
buffer, but many platforms provide an extended version
of getcwd that will allocate a buffer when called with
arguments (NULL, 0).

Display::Current() was depending upon this extended
behavior, which happens to work on Linux and Mac
platforms, but which doesn't work on other POSIX
platforms (such as Android.)

Fixed by using the preprocessor to choose between two
implementations of Display::Current, one of which depends
upon the extended semantics of getcwd, the other of which
only depends upon the standard semantics of getcwd.

BUG=

Committed: https://code.google.com/p/dart/source/detail?r=10309

[jackpal, 2012-08-06 20:34:45]

Hi Mads and Søren,

This is the first of several changes I will be submitting
to support building the Dart VM on Android.

I could use your advice on solving this issue.

The issue here is that the Directory::Current() method in directory_posix.cc is
assuming that getcwd(0, NULL) will
allocate space for the return value, but that behavior is
not part of the official POSIX standard for getcwd. Therefore Android, which
doesn't support that extension to
getcwd, is failing.

[siva, 2012-08-06 22:03:09]

I noticed that this change was submitted without a LGTM from the reviewers.
Normally we submit changes only after a LGTM from the reviewer.

It is ok to submit minor changes like changing the status of a test case etc.
with a TBR but not a code change.

[siva, 2012-08-06 22:07:13]

DBC:

One of the reasons why Ivan and I have been staunchly against having *_posix.cc
files is precisely this. There are always minor or major differences between
these platform and one ends with #if ... #endif stuff which makes the code
unreadable.

It seems always better to have _linux.cc _macos.cc and _xxx.cc files which makes
the code clear even if there is some repetition of code.

If the OS specific API is designed correctly the amount of code repetition will
be minimal.