Fixed strong mode errors and warnings reachable from lib/observe.dart

pubspec.yaml

 name: observe
 version: 0.13.3+1
 author: Polymer.dart Authors <web-ui-dev@dartlang.org>
 description: >
   Observable properties and objects for use in template_binding.
   Template Binding extends HTML and the DOM APIs to support a sensible
   separation between the UI (DOM) of a document or application and its
   underlying data (model). Updates to the model are reflected in the DOM and
   user input into the DOM is immediately assigned to the model.
 homepage: https://www.dartlang.org/polymer-dart/
 dependencies:
   analyzer: '^0.27.0'
   barback: '>=0.14.2 <0.16.0'
+  func: '>=0.1.0 <0.2.0'
   logging: '>=0.9.0 <0.12.0'
   path: '>=0.9.0 <2.0.0'
   smoke: '>=0.1.0 <0.4.0'
   source_maps: '>=0.9.4 <0.11.0'
   source_span: '>=1.0.0 <2.0.0'
   utf: '>=0.9.0 <0.10.0'
   code_transformers: '>=0.3.0 <0.5.0'
 dev_dependencies:
   benchmark_harness: '>=1.0.0 <2.0.0'
   browser: any
   chart: '>=1.0.8 <2.0.0'
   test: '^0.12.0'
   stack_trace: '>=0.9.1 <2.0.0'
 environment:
   sdk: '>=1.8.0 <2.0.0'
 transformers:
 - observe:
     files:
       - benchmark/test_observable.dart
       - benchmark/test_path_observable.dart