pkg/analyzer: fix formatter bin crash due to empty selection flag

pkg/analyzer/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 'package:args/args.dart';
@@ skipping 72 matching lines @@
       length = -1;
     } else {
       throw new FormatterException('Line length is specified as an Integer or '
           'the value "Inf".');
     }
   }
   return length;
 }
 
 
-Selection _parseSelection(selectionOption) {
-  if (selectionOption != null) {
-    var units = selectionOption.split(',');
-    if (units.length == 2) {
-      var offset = _toInt(units[0]);
-      var length = _toInt(units[1]);
-      if (offset != null && length != null) {
-        return new Selection(offset, length);
-      }
+Selection _parseSelection(String selectionOption) {
+  if(selectionOption == null) return null;
+
+  var units = selectionOption.split(',');
+  if (units.length == 2) {
+    var offset = _toInt(units[0]);
+    var length = _toInt(units[1]);
+    if (offset != null && length != null) {
+      return new Selection(offset, length);
     }
   }
   throw new FormatterException('Selections are specified as integer pairs '
                                '(e.g., "(offset, length)".');
 }
 
 int _toInt(str) => int.parse(str, onError: (_) => null);
 
 _formatPaths(paths) {
   paths.forEach((path) {
@@ skipping 116 matching lines @@
                      'offset': formatResult.selection.offset,
                      'length': formatResult.selection.length
                   }
     });
 
 /// Log the given [msg].
 _log(String msg) {
   //TODO(pquitslund): add proper log support
   print(msg);
 }