Remove support for the old Span classes from source_maps.

pkg/polymer/pubspec.yaml

 name: polymer
-version: 0.12.0+3
+version: 0.12.0+4
 author: Polymer.dart Authors <web-ui-dev@dartlang.org>
 description: >
   Polymer.dart is a new type of library for the web, built on top of Web
   Components, and designed to leverage the evolving web platform on modern
   browsers.
 homepage: https://www.dartlang.org/polymer-dart/
 dependencies:
   analyzer: '>=0.15.6 <0.16.0'
   args: '>=0.10.0 <0.13.0'
   barback: '>=0.14.2 <0.16.0'
   browser: '>=0.10.0 <0.11.0'
   code_transformers: '>=0.2.0 <0.3.0'
   html5lib: '>=0.11.0 <0.13.0'
   logging: '>=0.9.0 <0.10.0'
   observe: '>=0.11.0 <0.12.0'
   path: '>=0.9.0 <2.0.0'
   polymer_expressions: '>=0.12.0 <0.13.0'
   smoke: '>=0.2.0 <0.3.0'
-  source_maps: '>=0.9.0 <0.10.0'
+  source_maps: '>=0.9.0 <0.11.0'
   source_span: '>=1.0.0 <2.0.0'
   template_binding: '>=0.12.0 <0.13.0'
   web_components: '>=0.5.0 <0.6.0'
   yaml: '>=0.9.0 <3.0.0'
 dev_dependencies:
   unittest: '>=0.10.0 <0.11.0'
 transformers:
 - polymer/src/build/mirrors_remover:
     $include: lib/polymer.dart
 - observe:
     files: lib/src/instance.dart
     $include: lib/src/instance.dart
 environment:
   sdk: '>=1.4.0 <2.0.0'