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

benchmark/benchmark.dart

Index: benchmark/benchmark.dart
diff --git a/benchmark/benchmark.dart b/benchmark/benchmark.dart
index a468261ea976e3115776a619524809ce30f8b54b..3ab9d48272657fc864f93198592548a0c5b3364f 100644
--- a/benchmark/benchmark.dart
+++ b/benchmark/benchmark.dart
@@ -60,7 +60,7 @@ void formatSource() {
 
   // 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);
   }