Step one towards stable error messages with details:

pkg/code_transformers/test/messages_test.dart

+// 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:convert';
+import 'package:unittest/unittest.dart';
+import 'package:code_transformers/messages/messages.dart';
+import 'package:source_span/source_span.dart';
+
+main() {
+  group('snippet', () {
+    test('template with no-args works', () {
+      expect(new MessageTemplate(_id('code_transformers', 1),
+            'this message has no args', '', '').snippet,
+          'this message has no args');
+    });
+
+    test('template with args throws', () {
+      expect(() => new MessageTemplate(_id('code_transformers', 1),
+            'this message has %-args-%', '', '')
+          .snippet,
+          throws);
+    });
+
+    test('can pass arguments to create snippet', () {
+      expect(new MessageTemplate(_id('code_transformers', 1),
+            'a %-b-% c something %-name-% too', '', '')
+          .create({'b': "1", 'name': 'foo'}).snippet,
+          'a 1 c something foo too');
+    });
+  });
+
+  test('equals', () {
+      expect(new MessageId('hi', 23) == new MessageId('hi', 23), isTrue);
+      expect(new MessageId('foo', 23) != new MessageId('bar', 23), isTrue);
+      expect(new MessageId('foo', 22) != new MessageId('foo', 23), isTrue);
+  });
+
+  for (var encode in [true, false]) {
+    var toJson = encode 
+        ? (o) => o.toJson()
+        : (o) => JSON.decode(JSON.encode(o));
+    group('serialize/deserialize ${encode ? "and stringify": ""}', () {
+      test('message id', () {
+        _eq(msg) {
+          expect(new MessageId.fromJson(toJson(msg)) == msg, isTrue);
+        }
+        _eq(const MessageId('hi', 23));
+        _eq(new MessageId('hi', 23));
+        _eq(new MessageId('a_b', 23));
+        _eq(new MessageId('a-b', 23));
+        _eq(new MessageId('a-b-', 3));
+        _eq(new MessageId('a', 21));
+      });
+
+      test('message', () {
+        _eq(msg) {
+          var parsed = new Message.fromJson(toJson(msg));
+          expect(msg.id, parsed.id);
+          expect(msg.snippet, parsed.snippet);
+        }
+        _eq(new Message(_id('hi', 33), 'snippet here'));
+        _eq(new MessageTemplate(_id('hi', 33), 'snippet', 'ignored', 'ignored'));
+      });
+
+      test('log entry', () {
+        _eq(entry) {
+          var parsed = new BuildLogEntry.fromJson(toJson(entry));
+          expect(entry.message.id, parsed.message.id);
+          expect(entry.message.snippet, entry.message.snippet);
+          expect(entry.level, parsed.level);
+          expect(entry.span, parsed.span);
+        }
+        _eq(_entry(33, 'hi there', 12));
+        _eq(_entry(33, 'hi there-', 11));
+      });
+
+      test('log entry table', () {
+        var table = new LogEntryTable();
+        table.add(_entry(11, 'hi there', 23));
+        table.add(_entry(13, 'hi there', 21));
+        table.add(_entry(11, 'hi there', 26));
+        expect(table.entries.length, 2);
+        expect(table.entries[_id('hi', 11)].length, 2);
+        expect(table.entries[_id('hi', 13)].length, 1);
+
+        var table2 = new LogEntryTable.fromJson(toJson(table));
+        expect(table2.entries.length, 2);
+        expect(table2.entries[_id('hi', 11)].length, 2);
+        expect(table2.entries[_id('hi', 13)].length, 1);
+        expect(table2.entries[_id('hi', 11)][0].span,
+            table.entries[_id('hi', 11)][0].span);
+        expect(table2.entries[_id('hi', 11)][1].span,
+            table.entries[_id('hi', 11)][1].span);
+        expect(table2.entries[_id('hi', 13)][0].span,
+            table.entries[_id('hi', 13)][0].span);
+      });
+    });
+  }
+}
+_id(s, i) => new MessageId(s, i);
+_entry(id, snippet, offset) => new BuildLogEntry(
+    new Message(_id('hi', id), snippet),
+    new SourceSpan(
+      new SourceLocation(offset, sourceUrl: 'a', line: 1, column: 3),
+      new SourceLocation(offset + 2, sourceUrl: 'a', line: 1, column: 5),
+      'hi'),
+    'Warning');