Add external byte array API and finalize external strings and byte arrays.

Add external byte array API and finalize external strings and byte arrays.

This change adds peer and callback arguments to the external byte array
constructor and moves the peer, callback, and data storage into a VM
peer object.  At construction time, a weak persistent handle and VM
callback is associated with the VM peer.  The VM callback will delete
the VM peer and invokes the user callback to reclaim the user peer.

This change also activates the callback support for the various classes of
external strings.

Committed: https://code.google.com/p/dart/source/detail?r=4164

[siva, 2012-02-11 03:00:58]

LGTM after addressing the issue of deleting the WeakPersistentHandle when there
is no user specified callback by either arming the callback with a default
callback that deletes the handle or having two kinds of callbacks
(WeakPersistenHandle callback and an PeerCallback).

http://codereview.chromium.org/9368049/diff/1010/runtime/vm/dart_api_impl.cc
File runtime/vm/dart_api_impl.cc (right):

http://codereview.chromium.org/9368049/diff/1010/runtime/vm/dart_api_impl.cc#...
runtime/vm/dart_api_impl.cc:1656: DARTSCOPE(Isolate::Current());
Should we complain with error messages here for
 - data being NULL
 - length is a negative value (also for Dart_NewByteArray)

http://codereview.chromium.org/9368049/diff/1010/runtime/vm/object.cc
File runtime/vm/object.cc (right):

http://codereview.chromium.org/9368049/diff/1010/runtime/vm/object.cc#newcode...
runtime/vm/object.cc:7344: weak_ref->set_callback(callback);
As discussed offline we need a strategy to delete this WeakPersistentHandle in
the case when there is no user defined callback.

[cshapiro, 2012-02-11 04:11:49]

http://codereview.chromium.org/9368049/diff/1010/runtime/vm/dart_api_impl.cc
File runtime/vm/dart_api_impl.cc (right):

http://codereview.chromium.org/9368049/diff/1010/runtime/vm/dart_api_impl.cc#...
runtime/vm/dart_api_impl.cc:1656: DARTSCOPE(Isolate::Current());
Sure, done.  I think it is okay for data to be NULL when length is 0.  I have
added code to fail all other cases with a NULL data value and when length is
negative.

http://codereview.chromium.org/9368049/diff/1010/runtime/vm/object.cc
File runtime/vm/object.cc (right):

http://codereview.chromium.org/9368049/diff/1010/runtime/vm/object.cc#newcode...
runtime/vm/object.cc:7344: weak_ref->set_callback(callback);
Yes, done.  The strategy I chose was to add a new type for the
WeakPersistentHandle finalizer.  This simplified the code in several places. 
For example, the callback passed into this function will always delete the
handle it is called with.