Stop resolving all of js_helper unconditionally.

sdk/lib/_internal/compiler/implementation/lib/string_helper.dart

 // 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.
 
 part of _js_helper;
 
 class StringMatch implements Match {
   const StringMatch(int this.start,
                     String this.str,
                     String this.pattern);
@@ skipping 59 matching lines @@
 }
 
 stringReplaceJS(receiver, replacer, to) {
   // The JavaScript String.replace method recognizes replacement
   // patterns in the replacement string. Dart does not have that
   // behavior.
   to = JS('String', r"#.replace('$', '$$$$')", to);
   return JS('String', r'#.replace(#, #)', receiver, replacer, to);
 }
 
-final RegExp quoteRegExp = new JSSyntaxRegExp(r'[-[\]{}()*+?.,\\^$|#\s]');
+final RegExp quoteRegExp = const JSSyntaxRegExp(r'[-[\]{}()*+?.,\\^$|#\s]');
 
 stringReplaceAllUnchecked(receiver, from, to) {
   checkString(to);
   if (from is String) {
     if (from == "") {
       if (receiver == "") {
         return to;
       } else {
         StringBuffer result = new StringBuffer();
         int length = receiver.length;
@@ skipping 134 matching lines @@
   void addCharCode(int charCode) {
     add(new String.fromCharCodes([charCode]));
   }
 
   void clear() {
     _contents = "";
   }
 
   String toString() => _contents;
 }