Format selection preservation editor bits.

pkg/analyzer_experimental/bin/formatter.dart

 #!/usr/bin/env 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.
 
 import 'dart:convert';
 import 'dart:io';
 import 'dart:utf';
 
@@ skipping 141 matching lines @@
       CodeKind.COMPILATION_UNIT, src, selection: selection);
   if (machineFormat) {
     return _toJson(formatResult);
   }
   return formatResult.source;
 }
 
 _toJson(formatResult) => 
     // Actual JSON format TBD
     JSON.encode({'source': formatResult.source,
-                 'selection': formatResult.selection.toString()});
+                 'selection': {
+                     'offset': formatResult.selection.offset,
+                     'length': formatResult.selection.length
+                  }
+    });
 
 /// Log the given [msg].
 _log(String msg) {
   //TODO(pquitslund): add proper log support
   print(msg);
 }