chore: support any version of pkg/observe >= 0.13.1

Closes angular#3117

Author: kevmoo

modules/angular2/pubspec.yaml

@@ -19,7 +19,7 @@ dependencies:
   source_span: '^1.0.0'
   stack_trace: '^1.1.1'
   quiver: '^0.21.4'
-  observe: '0.13.1'
+  observe: '^0.13.1'
 dev_dependencies:
   guinness: '^0.1.17'
 transformers: