fix observe package to be strong mode clean

pubspec.yaml

 name: observe
-version: 0.13.4
+version: 0.13.5
 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'
+  analyzer: ^0.27.0
   barback: '>=0.14.2 <0.16.0'
-  func: '>=0.1.0 <0.2.0'
+  func: ^0.1.0
   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'
+  source_span: ^1.0.0
+  utf: ^0.9.0
+  code_transformers: ^0.4.2
 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'
+  sdk: ^1.9.0
 transformers:
 - observe:
     files:
       - benchmark/test_observable.dart
       - benchmark/test_path_observable.dart
 - test/pub_serve:
     $include: test/**_test.dart