Add CompilationUnit abstraction for tracking "project files".

dart/site/try/src/compilation_unit.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.
+
+library trydart.compilationUnit;
+
+import 'dart:async' show
+    Stream,
+    StreamController;
+
+class CompilationUnitData {
+  final String name;
+  String content;
+
+  CompilationUnitData(this.name, this.content);
+}
+
+class CompilationUnit extends CompilationUnitData {
+  // Extending [CompilationUnitData] allows this class to hide the storage
+  // allocated for [content] without introducing new names. The conventional
+  // way of acheiving this is to introduce a library-private field, but library
+  // privacy isn't without problems.
+
+  static StreamController<CompilationUnit> controller =
+      new StreamController<CompilationUnit>(sync: false);
+
+  static Stream<CompilationUnit> get onChanged => controller.stream;
+
+  CompilationUnit(String name, String content)
+      : super(name, content);
+
+  void set content(String newContent) {
+    if (content != newContent) {
+      super.content = newContent;
+      controller.add(this);
+    }
+  }
+}