Properly set the lower bound for packages' dependencies on source_maps.

pkg/observe/CHANGELOG.md

 # changelog
 
 This file contains highlights of what changes on each version of the observe
 package.
 
+#### Pub version 0.11.0+4
+  * Raise the lower bound on the source_maps constraint to exclude incompatible
+    versions.
+
 #### Pub version 0.11.0+3
   * Widen the constraint on source_maps.
 
 #### Pub version 0.11.0+2
   * Widen the constraint on barback.
 
 #### Pub version 0.11.0+1
   * Switch from `source_maps`' `Span` class to `source_span`'s `SourceSpan`
     class.
 
@@ skipping 20 matching lines @@
     for development time, but assumes frameworks (like polymer) and apps that
     use it directly will either generate code that replaces the use of mirrors,
     or add the @MirrorsUsed declaration themselves. For convinience, you can
     import 'package:observe/mirrors_used.dart', and that will add a @MirrorsUsed
     annotation that preserves properties and classes labeled with @reflectable
     and properties labeled with @observable.
   * Updated to match [observe-js#0152d54][0152d54]
 
 [0152d54]: https://github.com/Polymer/observe-js/blob/0152d542350239563d0f2cad39d22d3254bd6c2a/src/observe.js
 [e212e74]: https://github.com/Polymer/observe-js/blob/e212e7473962067c099a3d1859595c2f8baa36d7/src/observe.js