Handle null content in ContentChange.

pkg/analysis_server/lib/src/domain_analysis.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.
 
 library domain.analysis;
 
 import 'dart:collection';
 
 import 'package:analysis_server/src/analysis_server.dart';
 import 'package:analysis_server/src/constants.dart';
@@ skipping 86 matching lines @@
     }
     server.setAnalysisSubscriptions(subMap);
     return new Response(request.id);
   }
 
   Response updateContent(Request request) {
     var changes = new HashMap<String, ContentChange>();
     RequestDatum filesDatum = request.getRequiredParameter(FILES);
     filesDatum.forEachMap((file, changeDatum) {
       var change = new ContentChange();
-      change.content = changeDatum[CONTENT].asString();
+      change.content = changeDatum[CONTENT].isNull ? null :
+          changeDatum[CONTENT].asString();
       if (changeDatum.hasKey(OFFSET)) {
         change.offset = changeDatum[OFFSET].asInt();
         change.oldLength = changeDatum[OLD_LENGTH].asInt();
         change.newLength = changeDatum[NEW_LENGTH].asInt();
       }
       changes[file] = change;
     });
     server.updateContent(changes);
     return new Response(request.id);
   }
@@ skipping 12 matching lines @@
 
 /**
  * A description of the change to the content of a file.
  */
 class ContentChange {
   String content;
   int offset;
   int oldLength;
   int newLength;
 }