Use a pool of native ports for directory operations

Use a pool of native ports for directory operations

Each directory object creates a new service port which will start a
new thread. As these service ports are currently never closed this is
a huge problem causing the standalone VM to fail when running out of
threads.

This change uses a pool of native ports for directory operations. This
still ensures that the operations for each directory object are
serialized and limits the number of native ports allocated and thus
threads started.

Even when closing of native ports is possible we might want to keep
some kind of pool like this. The reason for this is that to actually
determine when the native port is not needed any more we need a
finalizer callback on the directory object.

R=ager@google.com

BUG=none
TEST=tests/standalone/src/ManyDirectoryOperationsTest.dart

Committed: https://code.google.com/p/dart/source/detail?r=4795

[Søren Gjesse, 2012-03-01 13:55:46]

I will do something similar for file if we can agree on this.

[Mads Ager (google), 2012-03-01 14:28:14]

LGTM

As discussed offline this should be combined with a number of other things:
thread pool in the VM for the native ports, close operation on native ports that
actually frees the resources used by the port, some heuristics in dart:io for
flushing of inactive native ports.