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

benchmark/benchmark.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.benchmark.benchmark;
 
 import 'dart:io';
 
 import 'package:dart_style/dart_style.dart';
 
@@ skipping 42 matching lines @@
 
   return result;
 }
 
 void formatSource() {
   var formatter = new DartFormatter();
   var result = formatter.format(source);
 
   // Sanity check to make sure the output is what we expect and to make sure
   // the VM doesn't optimize "dead" code away.
-  if (result.length != 29796) {
+  if (result.length != 29791) {
     print("Incorrect output (length ${result.length}):\n$result");
     exit(1);
   }
 }
 
 const source = r"""
 // Copyright (c) 2012, 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.
 
@@ skipping 802 matching lines @@
 /// Throws a [SolveFailure] if not.
 void _validateSdkConstraint(Pubspec pubspec) {
   if (pubspec.environment.sdkVersion.allows(sdk.version)) return;
 
   throw new BadSdkVersionException(
       pubspec.name,
       'Package ${pubspec.name} requires SDK version '
           '${pubspec.environment.sdkVersion} but the current SDK is ' '${sdk.version}.');
 }
 """;