Add API to provide a selection range, and return the updated selection after formatting.

lib/src/formatter_exception.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 dart_style.src.formatter_exception;
 
 import 'package:analyzer/analyzer.dart';
 import 'package:source_span/source_span.dart';
 
 /// Thrown when one or more errors occurs while parsing the code to be
@@ skipping 14 matching lines @@
       var file = new SourceFile(error.source.contents.data,
           url: error.source.fullName);
 
       var span = file.span(error.offset, error.offset + error.length);
       if (buffer.isNotEmpty) buffer.writeln();
       buffer.write(span.message(error.message, color: true));
     }
 
     return buffer.toString();
   }
+
+  String toString() => message();
 }