Add metadata to messages emitted by live tests.

lib/src/backend/message.dart

Index: lib/src/backend/message.dart
diff --git a/lib/src/backend/message.dart b/lib/src/backend/message.dart
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+++ b/lib/src/backend/message.dart
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+// Copyright (c) 2016, 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.
+
+/// A message emitted by a test.
+///
+/// A message encompasses any textual information that should be presented to
+/// the user. Reporters are encouraged to visually distinguish different message
+/// types.
+class Message {
+  final MessageType type;
+
+  final String text;
+
+  Message(this.type, this.text);
+
+  Message.print(this.text) : type = MessageType.print;
+  Message.skip(this.text) : type = MessageType.skip;
+}
+
+class MessageType {
+  /// A message explicitly printed by the user's test.
+  static const print = const MessageType._("print");
+
+  /// A message indicating that a test, or some portion of one, was skipped.
+  static const skip = const MessageType._("skip");
+
+  /// The name of the message type.
+  final String name;
+
+  factory MessageType.parse(String name) {
+    switch (name) {
+      case "print": return MessageType.print;
+      case "skip": return MessageType.skip;
+      default: throw new ArgumentError('Invalid message type "$name".');
+    }
+  }
+
+  const MessageType._(this.name);
+
+  String toString() => name;
+}