Add CompilationUnit abstraction for tracking "project files".

dart/site/try/src/editor.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.editor;
 
 import 'dart:html';
 
 import 'package:compiler/implementation/scanner/scannerlib.dart'
   show
@@ skipping 102 matching lines @@
   int type = anchorNode.nodeType;
   if (type != Node.TEXT_NODE) return null;
   Text text = anchorNode;
   var parent = text.parent;
   if (parent is! Element) return null;
   if (mainEditorPane == parent) return null;
   return parent;
 }
 
 bool isMalformedInput = false;
-String currentSource = "";
 
 addDiagnostic(String kind, String message, int begin, int end) {
   observer.disconnect();
   Selection selection = window.getSelection();
   int offset = 0;
   int anchorOffset = 0;
   bool hasSelection = false;
   Node anchorNode = selection.anchorNode;
   bool foundNode = false;
   void walk4(Node node) {
@@ skipping 85 matching lines @@
 
 diagnostic(text, tip) {
   if (text is String) {
     text = new Text(text);
   }
   return new AnchorElement()
       ..classes.add('diagnostic')
       ..append(text)
       ..append(tip);
 }