Step one towards stable error messages with details:

pkg/polymer/CHANGELOG.md

-# changelog
-
 This file contains highlights of what changes on each version of the polymer
 package. We will also note important changes to the polyfill packages (observe,
 web_components, and template_binding) if they impact polymer.
 
-#### Pub version 0.13.0+2
+#### 0.13.1-dev
+  * Upgraded error messages to have a unique and stable identifier. This
+    requires a version of `code_transformers` newer than `0.2.2-dev`.
+
+#### 0.13.0+2
   * Update internal transformers to delete .concat.js and .map files when in
     release mode, saving about 1MB of space in the built output.
 
-#### Pub version 0.13.0+1
+#### 0.13.0+1
   * Bug fix for http://dartbug.com/18171. Elements that extend other elements
     but don't have a template will still inherit styles from those elements.
   * Bug fix for http://dartbug.com/20544. Better runtime logging when attributes
     are defined on an element but have no corresponding property on the class.
 
-#### Pub version 0.13.0
+#### 0.13.0
   * Update to match polymer 0.3.5 ([polymer-dev#5d00e4b][5d00e4b]). There was a
     breaking change in the web_components package where selecting non-rendered 
     elements doesn't work, but it shouldn't affect most people. See 
     https://github.com/Polymer/ShadowDOM/issues/495.
 
-#### Pub version 0.12.2+1
+#### 0.12.2+1
   * Small bug fix for `polymer:new_element`
 
-#### Pub version 0.12.2
+#### 0.12.2
   * Fix for [20539](http://dartbug.com/20539). Log widget will now html escape
     messages.
   * Fix for [20538](http://dartbug.com/20538). Log widget will now surface lint
     logs from imported files.
   * Backward compatible change to prepare for upcoming change of the user agent
     in Dartium.
   * `pub run polymer:new_element` now supports specifying a base class.
     **Note**: only native DOM types and custom elements written in Dart can be
     extended. Elements adapted from Javascript (like core- and paper- elements)
     cannot be extended.
   * other bug fixes in `polymer:new_entry`.
 
-#### Pub version 0.12.1
+#### 0.12.1
   * **New**: When running in pub-serve, any warnings and errors detected by the
     polymer transformers will be displayed in the lower-right corner of your
     entrypoint page. You can opt-out by adding this option to your pubspec:
 
         transformers:
         - polymer:
             ...
             inject_build_logs_in_output: false
 
   * **New**: there are now two template generators in the polymer package! On
@@ skipping 25 matching lines @@
             inline_stylesheets:
                 default: false
                 web/foo.css: true
                 packages/foo/bar.css: true
 
     
   * Bug fix for http://dartbug.com/20286. Bindings in url attributes will no
     longer throw an error.
 
 
-#### Pub version 0.12.0+7
+#### 0.12.0+7
   * Widen the constraint on `unittest`.
 
-#### Pub version 0.12.0+6
+#### 0.12.0+6
   * Widen the constraint on analyzer.
   * Support for `_src` and similar attributes in polymer transformers.
 
-#### Pub version 0.12.0+5
+#### 0.12.0+5
   * Raise the lower bound on the source_maps constraint to exclude incompatible
     versions.
 
-#### Pub version 0.12.0+4
+#### 0.12.0+4
   * Widen the constraint on source_maps.
 
-#### Pub version 0.12.0+3
+#### 0.12.0+3
   * Fix a final use of `getLocationMessage`.
 
-#### Pub version 0.12.0+2
+#### 0.12.0+2
   * Widen the constraint on barback.
 
-#### Pub version 0.12.0+1
+#### 0.12.0+1
   * Switch from `source_maps`' `Span` class to `source_span`'s `SourceSpan`
     class.
 
-#### Pub version 0.12.0
+#### 0.12.0
  * Updated to match polymer 0.3.4 ([polymer-dev#6ad2d61][6ad2d61]), this
    includes the following changes:
      * added @ComputedProperty
      * @published can now be written using the readValue/writeValue helper
        methods to match the same timing semantics as Javscript properties.
      * underlying packages are also updated. Some noticeable changes are:
        * observe: path-observers syntax is slightly different
        * polymer_expressions: updating the value of an expression will issue a
          notification.
        * template_binding: better NodeBind interop support (for
          two-way bindings with JS polymer elements).
  * Several fixes for CSP, including a cherry-pick from polymer.js
    [commit#3b690ad][3b690ad].
  * Fix for [17596](https://code.google.com/p/dart/issues/detail?id=17596)
  * Fix for [19770](https://code.google.com/p/dart/issues/detail?id=19770)
 
-#### Pub version 0.11.0+5
+#### 0.11.0+5
   * fixes web_components version in dependencies
 
-#### Pub version 0.11.0+4
+#### 0.11.0+4
   * workaround for bug
     [19653](https://code.google.com/p/dart/issues/detail?id=19653)
 
-#### Pub version 0.11.0+3
+#### 0.11.0+3
   * update readme
 
-#### Pub version 0.11.0+2
+#### 0.11.0+2
   * bug fix: event listeners were not in the dirty-checking zone
   * bug fix: dispatch event in auto-binding
 
-#### Pub version 0.11.0+1
+#### 0.11.0+1
   * Added a workaround for bug in HTML imports (issue
     [19650](https://code.google.com/p/dart/issues/detail?id=19650)).
 
-#### Pub version 0.11.0
+#### 0.11.0
   * **breaking change**: platform.js and dart_support.js must be specified in
     your entry points at the beginning of `<head>`.
   * **breaking change**: polymer.html is not required in entrypoints, but it is
     required from files that use `<polymer-element>`.
   * **breaking change**: enteredView/leftView were renamed to attached/detached.
     The old lifecycle methods will not be invoked.
   * **breaking change**: Event bindings with `@` are no longer supported.
   * **breaking change**: `@published` by default is no longer reflected as an
     attribute by default. This might break if you try to use the attribute in
     places like CSS selectors. To make it reflected back to an attribute use
     `@PublishedProperty(reflect: true)`.
 
-#### Pub version 0.10.1
+#### 0.10.1
   * Reduce the analyzer work by mocking a small subset of the core libraries.
 
-#### Pub version 0.10.0+1
+#### 0.10.0+1
   * Better error message on failures in pub-serve/pub-build when pubspec.yaml
     is missing or has a wrong configuration for the polymer transformers.
 
-#### Pub version 0.10.0
+#### 0.10.0
   * Interop with polymer-js elements now works.
   * Polymer polyfills are now consolidated in package:web_components, which is
     identical to platform.js from http://polymer-project.org.
   * The output of pub-build no longer uses mirrors. We replace all uses of
     mirrors with code generation.
   * **breaking change**: Declaring a polymer app requires an extra import to
     `<link rel="import" href="packages/polymer/polymer.html">`
   * **breaking change**: "noscript" polymer-elements are created by polymer.js,
     and therefore cannot be extended (subtyped) in Dart. They can still be used
     by Dart elements or applications, however.
   * New feature: `@ObserveProperty('foo bar.baz') myMethod() {...}` will cause
     myMethod to be called when "foo" or "bar.baz" changes.
   * Updated for 0.10.0-dev package:observe and package:template_binding changes.
   * **breaking change**: @initMethod and @CustomTag are only supported on
     public classes/methods.
 
-#### Pub version 0.9.5
+#### 0.9.5
   * Improvements on how to handle cross-package HTML imports.
 
-#### Pub version 0.9.4
+#### 0.9.4
   * Removes unused dependency on csslib.
 
-#### Pub version 0.9.3+3
+#### 0.9.3+3
   * Removes workaround now that mirrors implement a missing feature. Requires
     SDK >= 1.1.0-dev.5.0.
 
-#### Pub version 0.9.3+2
+#### 0.9.3+2
   * Fix rare canonicalization bug
     [15694](https://code.google.com/p/dart/issues/detail?id=15694)
 
-#### Pub version 0.9.3+1
+#### 0.9.3+1
   * Fix type error in runner.dart
     [15649](https://code.google.com/p/dart/issues/detail?id=15649).
 
-#### Pub version 0.9.3
+#### 0.9.3
   * pub-build now runs the linter automatically
 
-#### Pub version 0.9.2+4
+#### 0.9.2+4
   * fix linter on SVG and MathML tags with XML namespaces
 
-#### Pub version 0.9.2+3
+#### 0.9.2+3
   * fix [15574](https://code.google.com/p/dart/issues/detail?id=15574),
     event bindings in dart2js, by working around issue
     [15573](https://code.google.com/p/dart/issues/detail?id=15573)
 
-#### Pub version 0.9.2+2
+#### 0.9.2+2
   * fix enteredView in dart2js, by using custom_element >= 0.9.1+1
 
 [6ad2d61]:https://github.com/Polymer/polymer-dev/commit/6a3e1b0e2a0bbe546f6896b3f4f064950d7aee8f
 [3b690ad]:https://github.com/Polymer/polymer-dev/commit/3b690ad0d995a7ea339ed601075de2f84d92bafd