Sky/Effen refactor: all nodes receive _mount/_unmount signal

Sky/Effen refactor: all nodes receive _mount/_unmount signal.

This patch refactors Effen in advance of allowing a final transform step from Effen nodes to sky nodes. The central changes here are:

1) Effen nodes get a _parentNode pointer
2) The lifetime of effen nodes is
  ->_sync (0 to N times)
     -> the first _sync() causes a _mount()
  ->_unmount
3) Node should expect to sync even when they are first inserted (in which case they sync against a prototypical empty node which their class must provide).
4) Subclasses now override _syncNode() no longer takes host & insertBefore (_mount does). In the one case that a node must be replaced without being unmounted (inside _syncInternal), the old node is inspected for it's sky position.

TBR=abarth

Committed: https://chromium.googlesource.com/external/mojo/+/23cea7f0dfb1469d34afec2baa6d52ddb580e4da

[rafaelw, 2015-03-17 00:29:14]

rafaelw@chromium.org changed reviewers:
+ abarth@chromium.org

[rafaelw, 2015-03-17 03:01:21]

Committed patchset #5 (id:80001) manually as
23cea7f0dfb1469d34afec2baa6d52ddb580e4da (presubmit successful).

[abarth-chromium, 2015-03-17 03:23:40]

lgtm

https://codereview.chromium.org/996103008/diff/80001/sky/framework/fn.dart
File sky/framework/fn.dart (right):

https://codereview.chromium.org/996103008/diff/80001/sky/framework/fn.dart#ne...
sky/framework/fn.dart:152: static Text _emptyText = new Text(null);
final

https://codereview.chromium.org/996103008/diff/80001/sky/framework/fn.dart#ne...
sky/framework/fn.dart:284: // print("---Syncing children of $_key");
Do we want to leave these prints in?

[rafaelw, 2015-03-18 19:08:32]

https://codereview.chromium.org/996103008/diff/80001/sky/framework/fn.dart
File sky/framework/fn.dart (right):

https://codereview.chromium.org/996103008/diff/80001/sky/framework/fn.dart#ne...
sky/framework/fn.dart:152: static Text _emptyText = new Text(null);
On 2015/03/17 03:23:40, abarth wrote:
> final

fixed in https://codereview.chromium.org/1009543008

https://codereview.chromium.org/996103008/diff/80001/sky/framework/fn.dart#ne...
sky/framework/fn.dart:284: // print("---Syncing children of $_key");
On 2015/03/17 03:23:40, abarth wrote:
> Do we want to leave these prints in?

fixed in https://codereview.chromium.org/1009543008