Update charted to 0.4.8 and roll

packages/observe/pubspec.yaml

-name: observe
-version: 0.13.1+3
-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: '>=0.15.6 <0.27.0'
-  barback: '>=0.14.2 <0.16.0'
-  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: '>=0.2.3 <0.3.0'
-dev_dependencies:
-  benchmark_harness: '>=1.0.0 <2.0.0'
-  browser: any
-  chart: '>=1.0.8 <2.0.0'
-  unittest: '>=0.10.0 <0.12.0'
-  stack_trace: '>=0.9.1 <2.0.0'
-environment:
-  sdk: '>=1.2.0 <2.0.0'
-transformers:
-- observe:
-    files:
-      - benchmark/test_observable.dart
-      - benchmark/test_path_observable.dart