Use String.replaceRange() instead of concatenation.

pkg/analysis_server/lib/src/protocol.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 protocol;
 
 import 'dart:collection';
 import 'dart:convert';
 
 part 'generated_protocol.dart';
@@ skipping 55 matching lines @@
 
 
 /**
  * Get the result of applying the edit to the given [code].  Access via
  * SourceEdit.apply().
  */
 String _applyEdit(String code, SourceEdit edit) {
   if (edit.length < 0) {
     throw new RangeError('length is negative');
   }
-  return code.substring(0, edit.offset) +
-      edit.replacement +
-      code.substring(edit.end);
+  return code.replaceRange(edit.offset, edit.end, edit.replacement);
 }
 
 /**
  * Get the result of applying a set of [edits] to the given [code].  Edits
  * are applied in the order they appear in [edits].  Access via
  * SourceEdit.applySequence().
  */
 String _applySequence(String code, Iterable<SourceEdit> edits) {
   edits.forEach((SourceEdit edit) {
     code = edit.apply(code);
@@ skipping 844 matching lines @@
     hash = 0x1fffffff & (hash + ((0x03ffffff & hash) << 3));
     hash = hash ^ (hash >> 11);
     return 0x1fffffff & (hash + ((0x00003fff & hash) << 15));
   }
 
   static int hash2(a, b) => finish(combine(combine(0, a), b));
 
   static int hash4(a, b, c, d) =>
       finish(combine(combine(combine(combine(0, a), b), c), d));
 }