title: Observer reaction functions should not change model
body: <p>Chapter_06 in AngularDart example
  when I run it in Dartium, there are some error:</p>
  
  <pre><code>Observer reaction functions should not change model.
  These watch changes were detected: ctrl.allRecipes: [Instance of 'Recipe', Instance of 'Recipe']
  cmp.categories: [Appetizers, Salads, Soups, Main Dishes, Side Dishes, Desserts] 
  &lt;= [Appetizers, Salads, Soups, Main Dishes, Side Dishes, Desserts]
  These observe changes were detected:
  
  STACKTRACE:
  #0      RootScope.flush.&lt;anonymous closure&gt; (package:angular/core/scope.dart:554:11)
  #1      RootScope.flush (package:angular/core/scope.dart:560:9)
  #2      RootScope.flush (package:angular/core/scope.dart:561:7)
  #3      RootScope.flush (package:angular/core/scope.dart:561:7)
  #4      apply (package:angular/core/scope.dart:262:18)
  #5      _rootRun (dart:async/zone.dart:710)
  #6      _rootRun (dart:async/zone.dart:711)
  #7      _rootRun (dart:async/zone.dart:711)
  #8      _ZoneDelegate.run (dart:async/zone.dart:440)
  #9      NgZone._finishTurn (package:angular/core/zone.dart:96:21)
  </code></pre>
  
  <p>How to fix it?</p>