Change Detection: perf, fix, style

Edit: The 2 lasts commits are about improving perfs:
1. by using regular fns over getters/setters. The speed gain is impressive in FF, Chrome is a little bit slower but still much faster than FF,
2. by inlining `isPresent()` and `isBlank()` where there is no need to test for `undefined`

Note: benchpress results sometimes vary from a run to the next, especially with FF. 
I actually prefer using isPresent and isBlank cause it gets rid of the bugs related to undefined. For example, declared unassigned fields are undefined in JS and null in Dart.

What kind of performance gain did the change detection benchmark show?