Step one towards stable error messages with details:

pkg/observe/CHANGELOG.md

-# changelog
+#### 0.12.1-dev
+  * Upgraded error messages to have a unique and stable identifier.
 
-This file contains highlights of what changes on each version of the observe
-package.
-
-#### Pub version 0.12.0
+#### 0.12.0
   * Old transform.dart file removed. If you weren't use it it, this change is
     backwards compatible with version 0.11.0.
 
-#### Pub version 0.11.0+5
+#### 0.11.0+5
   * Widen the constraint on analyzer.
 
-#### Pub version 0.11.0+4
+#### 0.11.0+4
   * Raise the lower bound on the source_maps constraint to exclude incompatible
     versions.
 
-#### Pub version 0.11.0+3
+#### 0.11.0+3
   * Widen the constraint on source_maps.
 
-#### Pub version 0.11.0+2
+#### 0.11.0+2
   * Widen the constraint on barback.
 
-#### Pub version 0.11.0+1
+#### 0.11.0+1
   * Switch from `source_maps`' `Span` class to `source_span`'s `SourceSpan`
     class.
 
-#### Pub version 0.11.0
+#### 0.11.0
   * Updated to match [observe-js#e212e74][e212e74] (release 0.3.4)
   * ListPathObserver has been deprecated  (it was deleted a while ago in
     observe-js). We plan to delete it in a future release. You may copy the code
     if you still need it.
   * PropertyPath now uses an expression syntax including indexers. For example,
     you can write `a.b["m"]` instead of `a.b.m`.
   * **breaking change**: PropertyPath no longer allows numbers as fields, you
     need to use indexers instead. For example, you now need to write `a[3].d`
     instead of `a.3.d`.
   * **breaking change**: PathObserver.value= no longer discards changes (this is
     in combination with a change in template_binding and polymer to improve
     interop with JS custom elements).
 
-#### Pub version 0.10.0+3
+#### 0.10.0+3
   * minor changes to documentation, deprecated `discardListChages` in favor of
     `discardListChanges` (the former had a typo).
 
-#### Pub version 0.10.0
+#### 0.10.0
   * package:observe no longer declares @MirrorsUsed. The package uses mirrors
     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