Step one towards stable error messages with details:

pkg/polymer/test/build/log_injector_test.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import 'dart:async';
 import 'dart:html';
 import 'package:unittest/unittest.dart';
 import 'package:unittest/html_config.dart';
 import 'package:polymer/src/build/log_injector.dart';
 
 
 main() {
 
   useHtmlConfiguration();
 
   setUp(() => new LogInjector().injectLogs(
-      '''[
-          {"level": "Info", "message": "foo"},
-          {"level": "Warning", "message": "bar"},
-          {"level": "Error", "message": "baz"}
-      ]'''
+      '''{
+        "polymer#0":[{
+            "level":"Info",
+            "message":{"id":"polymer#0","snippet":"foo"}}
+        ],
+        "polymer#1":[{
+            "level":"Info",
+            "message":{"id":"polymer#1","snippet":"foo"},
+            "span":{
+              "start":{
+                "url":"web/test.html",
+                "offset":22,
+                "line":1,
+                "column":0
+              },
+              "end":{
+                "url":"web/test.html",
+                "offset":50,
+                "line":1,
+                "column":28
+              },
+              "text":"<polymer-element name=\\"x-a\\">"
+            }
+          }],
+        "polymer#2":[
+            {"level":"Warning","message":{"id":"polymer#2","snippet":"bar"}},
+            {"level":"Warning","message":{"id":"polymer#2",
+             "snippet":"bar again"}},
+            {"level":"Error","message":{"id":"polymer#2","snippet":"baz1"}}
+        ],
+        "foo#44":[{"level":"Error","message":{"id":"foo#44","snippet":"baz2"}}]
+      }'''
   ));
 
   test('can inject a functioning log widget', () {
     var logsElement = document.querySelector(".build-logs");
     expect(logsElement, isNotNull);
 
     var menuElements = logsElement.querySelectorAll(".menu > div");
     expect(menuElements.length, 3);
     var contentElements = logsElement.querySelectorAll(".content > div");
     expect(contentElements.length, 3);
 
     var expectedClasses = ['info', 'warning', 'error'];
 
     // Check initial setup.
     for (var i = 0; i < menuElements.length; ++i) {
       expect(menuElements[i].classes.contains(expectedClasses[i]), true);
       expect(menuElements[i].classes.contains('active'), false);
       expect(contentElements[i].classes.contains(expectedClasses[i]), true);
       expect(contentElements[i].classes.contains('active'), false);
-      expect(contentElements[i].querySelectorAll('.log').length, 1);
+      expect(contentElements[i].querySelectorAll('.log').length, 2);
     }
 
     // Test clicking each of the tabs.
     for (var i = 0; i < menuElements.length; ++i) {
       menuElements[i].click();
       for (var j = 0; j < menuElements.length; ++j) {
         expect(menuElements[j].classes.contains('active'), j == i);
         expect(contentElements[j].classes.contains('active'), j == i);
       }
     }
 
     // Test toggling same tab.
     expect(menuElements[2].classes.contains('active'), true);
     menuElements[2].click();
     expect(menuElements[2].classes.contains('active'), false);
     expect(contentElements[2].classes.contains('active'), false);
   });
 }