Step one towards stable error messages with details:

pkg/observe/test/unique_message_test.dart

Index: pkg/observe/test/unique_message_test.dart
diff --git a/pkg/observe/test/unique_message_test.dart b/pkg/observe/test/unique_message_test.dart
new file mode 100644
index 0000000000000000000000000000000000000000..54fb5a4629b34319c7a34a16a25543c251d02f52
--- /dev/null
+++ b/pkg/observe/test/unique_message_test.dart
@@ -0,0 +1,37 @@
+// Copyright (c) 2014, 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.
+
+/// Tests for some of the utility helper functions used by the compiler.
+library polymer.test.build.messages_test;
+
+import 'dart:mirrors';
+
+import 'package:unittest/unittest.dart';
+import 'package:code_transformers/messages/messages.dart' show Message;
+
+import 'package:observe/src/messages.dart' as p1;
+
+/// [p1] is accessed via mirrors, this comment prevents the analyzer from
+/// complaining about it.
+main() {
+  test('each message id is unique', () {
+    var seen = {};
+    int total = 0;
+    var mirrors = currentMirrorSystem();
+    var lib = mirrors.findLibrary(#observe.src.messages);
+    expect(lib, isNotNull);
+    lib.declarations.forEach((symbol, decl) {
+      if (decl is! VariableMirror) return;
+      var field = lib.getField(symbol).reflectee;
+      var name = MirrorSystem.getName(symbol);
+      if (field is! Message) return;
+      var id = field.id;
+      expect(seen.containsKey(id), isFalse, reason: 'Duplicate id `$id`. '
+          'Currently set for both `$name` and `${seen[id]}`.');
+      seen[id] = name;
+      total++;
+    });
+    expect(seen.length, total);
+  });
+}