Step one towards stable error messages with details:

pkg/observe/test/transformer_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 'package:barback/barback.dart';
 import 'package:observe/transformer.dart';
 import 'package:unittest/compact_vm_config.dart';
 import 'package:unittest/unittest.dart';
+import 'package:stack_trace/stack_trace.dart';
 
 main() {
   useCompactVMConfiguration();
 
   group('replaces Observable for ChangeNotifier', () {
     _testClause('extends Observable', 'extends ChangeNotifier');
     _testClause('extends Base with Observable',
         'extends Base with ChangeNotifier');
     _testClause('extends Base<T> with Observable',
         'extends Base<T> with ChangeNotifier');
@@ skipping 102 matching lines @@
       var end = output.indexOf(';', begin);
       if (end == -1) end = output.length;
       var init = output.substring(begin, end).trim().replaceAll('  ', ' ');
       expect(init, expected);
     });
   });
 }
 
 /// Helper that applies the transform by creating mock assets.
 Future<String> _transform(String code) {
-  var id = new AssetId('foo', 'a/b/c.dart');
-  var asset = new Asset.fromString(id, code);
-  var transformer = new ObservableTransformer();
-  return transformer.isPrimary(asset).then((isPrimary) {
-    expect(isPrimary, isTrue);
-    var transform = new _MockTransform(asset);
-    return transformer.apply(transform).then((_) {
-      expect(transform.outs, hasLength(1));
-      expect(transform.outs[0].id, id);
-      return transform.outs.first.readAsString();
+  return Chain.capture(() {
+    var id = new AssetId('foo', 'a/b/c.dart');
+    var asset = new Asset.fromString(id, code);
+    var transformer = new ObservableTransformer();
+    return transformer.isPrimary(asset).then((isPrimary) {
+      expect(isPrimary, isTrue);
+      var transform = new _MockTransform(asset);
+      return transformer.apply(transform).then((_) {
+        expect(transform.outs, hasLength(2));
+        expect(transform.outs[0].id, id);
+        expect(transform.outs[1].id, id.addExtension('._buildLogs.1'));
+        return transform.outs.first.readAsString();
+      });
     });
   });
 }
 
 class _MockTransform implements Transform {
   bool shouldConsumePrimary = false;
   List<Asset> outs = [];
   Asset _asset;
   TransformLogger logger = new TransformLogger(_mockLogFn);
   Asset get primaryInput => _asset;
 
   _MockTransform(this._asset);
   Future<Asset> getInput(AssetId id) {
     if (id == primaryInput.id) return new Future.value(primaryInput);
     fail('_MockTransform fail');
   }
 
   void addOutput(Asset output) {
     outs.add(output);
   }
 
   void consumePrimary() {
     shouldConsumePrimary = true;
   }
 
   readInput(id) => throw new UnimplementedError();
   readInputAsString(id, {encoding}) => throw new UnimplementedError();
-  hasInput(id) => throw new UnimplementedError();
+  hasInput(id) =>
+      new Future.value(id == _asset.id || outs.any((a) => a.id == id));
 
   static void _mockLogFn(AssetId asset, LogLevel level, String message,
                          span) {
     // Do nothing.
   }
 }
 
 String _sampleObservable(String annotation) => '''
 library A_foo;
 import 'package:observe/observe.dart';
@@ skipping 35 matching lines @@
     "      Foo\n"
     "          get foo => __\$foo; Foo __\$foo/*D*/= 1; "
         "${_makeSetter('Foo', 'foo')} "
         "@reflectable @$meta @otherMetadata Foo get bar => __\$bar; "
         "Foo __\$bar =/*A*/2/*B*/; ${_makeSetter('Foo', 'bar')}\n"
     "          @reflectable @$meta @otherMetadata Foo get quux => __\$quux; "
         "Foo __\$quux/*C*/; ${_makeSetter('Foo', 'quux')}\n\n"
     "  @reflectable @$meta dynamic get baz => __\$baz; dynamic __\$baz; "
         "${_makeSetter('dynamic', 'baz')}\n"
     "}\n";