Step one towards stable error messages with details:

pkg/observe/lib/src/messages.dart

Index: pkg/observe/lib/src/messages.dart
diff --git a/pkg/observe/lib/src/messages.dart b/pkg/observe/lib/src/messages.dart
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+++ b/pkg/observe/lib/src/messages.dart
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+// 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.
+
+/// Contains all warning messages produced by the observe transformer.
+library observe.src.messages;
+
+import 'package:code_transformers/messages/messages.dart';
+
+const NO_OBSERVABLE_ON_LIBRARY = const MessageTemplate(
+    const MessageId('observe', 1),
+    '@observable on a library no longer has any effect. '
+    'Instead, annotate individual fields as @observable.',
+    '`@observable` not supported on libraries',
+    _COMMON_MESSAGE_WHERE_TO_USE_OBSERVABLE);
+
+const NO_OBSERVABLE_ON_TOP_LEVEL = const MessageTemplate(
+    const MessageId('observe', 2),
+    'Top-level fields can no longer be observable. '
+    'Observable fields must be in observable objects.',
+    '`@observable` not supported on top-level fields',
+    _COMMON_MESSAGE_WHERE_TO_USE_OBSERVABLE);
+
+const NO_OBSERVABLE_ON_CLASS = const MessageTemplate(
+    const MessageId('observe', 3),
+    '@observable on a class no longer has any effect. '
+    'Instead, annotate individual fields as @observable.',
+    '`@observable` not supported on classes',
+    _COMMON_MESSAGE_WHERE_TO_USE_OBSERVABLE);
+
+const NO_OBSERVABLE_ON_STATIC_FIELD = const MessageTemplate(
+    const MessageId('observe', 4),
+    'Static fields can no longer be observable. '
+    'Observable fields must be in observable objects.',
+    '`@observable` not supported on static fields',
+    _COMMON_MESSAGE_WHERE_TO_USE_OBSERVABLE);
+
+const REQUIRE_OBSERVABLE_INTERFACE = const MessageTemplate(
+    const MessageId('observe', 5),
+    'Observable fields must be in observable objects. '
+    'Change this class to extend, mix in, or implement Observable.',
+    '`@observable` field not in an `Observable` class',
+    _COMMON_MESSAGE_WHERE_TO_USE_OBSERVABLE);
+
+const String _COMMON_MESSAGE_WHERE_TO_USE_OBSERVABLE = '''
+Only instance fields on `Observable` classes can be observable,
+and you must explicitly annotate each observable field as `@observable`.
+
+Support for using the `@observable` annotation in libraries, classes, and
+elsewhere is deprecated.
+''';