Use static final empty Modifier instead of null.

lib/compiler/implementation/dart_backend/backend.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.
 
 const bool REMOVE_ASSERTS = false;
 
 class ElementAst {
   final Node ast;
   final TreeElements treeElements;
 
@@ skipping 342 matching lines @@
       constructor.type = new FunctionType(
           compiler.types.voidType, const EmptyLink<DartType>(),
           constructor);
       constructor.cachedNode = new FunctionExpression(
           new Send(receiver: classElement.parseNode(compiler).name,
                    selector: synthesizedIdentifier),
           new NodeList(beginToken: new StringToken(OPEN_PAREN_INFO, '(', -1),
                        endToken: new StringToken(CLOSE_PAREN_INFO, ')', -1),
                        nodes: const EmptyLink<Node>()),
           new EmptyStatement(new StringToken(SEMICOLON_INFO, ';', -1)),
-          null, null, null, null);
+          null, Modifiers.EMPTY, null, null);
 
       classMembers[classElement].add(constructor);
       elementAsts[constructor] =
           new ElementAst(constructor.cachedNode, new TreeElementMapping());
     }
 
     // Create all necessary placeholders.
     PlaceholderCollector collector =
         new PlaceholderCollector(compiler, fixedMemberNames, elementAsts);
     // Add synthesizedIdentifier to set of unresolved names to rename it to
@@ skipping 141 matching lines @@
 }
 
 compareElements(e0, e1) {
   int result = compareBy((e) => e.getLibrary().uri.toString())(e0, e1);
   if (result != 0) return result;
   return compareBy((e) => e.position().charOffset)(e0, e1);
 }
 
 List<Element> sortElements(Collection<Element> elements) =>
     sorted(elements, compareElements);