Devoncarew fix startup

Fix an exception during startup. We were returning `null` from a method that callers expected to have return `bool`s.

@ussuri, @dinhviethoa

Uncaught Error: type 'Null' is not a subtype of type 'bool' of 'boolean expression'.
Stack Trace: 
#0      dirtyCheckObservables (package:observe/src/dirty_check.dart:61:36)
#1      Observable.dirtyCheck (package:observe/src/observable.dart:35:52)
#2      SparkPolymerUI.refreshFromModel (chrome-extension://kcjgcakhgelcejampmijgkjkadfcncjl/spark_polymer_ui.dart:71:26)
#3      SparkPolymerUI.modelReady (chrome-extension://kcjgcakhgelcejampmijgkjkadfcncjl/spark_polymer_ui.dart:60:21)