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. 
Dart2JS or JS? 

I ran the benchmark in JS with/without the commit (a dozen times), and I cannot see the difference.  

My version of Chrome: 41.0.2238.0 canary
I use perf_launch_chrome.sh to start Chrome with the right flags.