Use static final empty Modifier instead of null.

lib/compiler/implementation/resolver.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.
 
 abstract class TreeElements {
   Element operator[](Node node);
   Selector getSelector(Send send);
   DartType getType(TypeAnnotation annotation);
   bool isParameterChecked(Element element);
 }
 
 class TreeElementMapping implements TreeElements {
   final Element currentElement;
   final Map<Node, Element> map;
   final Map<Node, Selector> selectors;
   final Map<TypeAnnotation, DartType> types;
   final Set<Element> checkedParameters;
 
   TreeElementMapping([Element this.currentElement])
       : map = new LinkedHashMap<Node, Element>(),
         selectors = new LinkedHashMap<Node, Selector>(),
         types = new LinkedHashMap<TypeAnnotation, DartType>(),
         checkedParameters = new Set<Element>();
 
   operator []=(Node node, Element element) {
     assert(invariant(node, () {
-      if (node is FunctionExpression && node.modifiers != null) {
+      if (node is FunctionExpression) {
         return !node.modifiers.isExternal();
       }
       return true;
     }));
     assert(invariant(node, () {
       if (!element.isErroneous() && currentElement != null && element.isPatch) {
         return currentElement.getImplementationLibrary().isPatch;
       }
       return true;
     }));
@@ skipping 1197 matching lines @@
 
   visitFunctionExpression(FunctionExpression node) {
     visit(node.returnType);
     SourceString name;
     if (node.name === null) {
       name = const SourceString("");
     } else {
       name = node.name.asIdentifier().source;
     }
     FunctionElement enclosing = new FunctionElement.node(
-        name, node, ElementKind.FUNCTION, new Modifiers.empty(),
+        name, node, ElementKind.FUNCTION, Modifiers.EMPTY,
         scope.element);
     setupFunction(node, enclosing);
     defineElement(node, enclosing, doAddToScope: node.name !== null);
 
     // Run the body in a fresh statement scope.
     StatementScope oldScope = statementScope;
     statementScope = new StatementScope();
     visit(node.body);
     statementScope = oldScope;
 
@@ skipping 1042 matching lines @@
     SynthesizedConstructorElement constructor =
       new SynthesizedConstructorElement(element);
     element.addToScope(constructor, compiler);
     DartType returnType = compiler.types.voidType;
     constructor.type = new FunctionType(returnType, const EmptyLink<DartType>(),
                                         constructor);
     constructor.cachedNode =
       new FunctionExpression(new Identifier(element.position()),
                              new NodeList.empty(),
                              new Block(new NodeList.empty()),
-                             null, null, null, null);
+                             null, Modifiers.EMPTY, null, null);
   }
 
   isBlackListed(DartType type) {
     LibraryElement lib = element.getLibrary();
     return
       lib !== compiler.coreLibrary &&
       lib !== compiler.coreImplLibrary &&
       lib !== compiler.jsHelperLibrary &&
       (type.element === compiler.dynamicClass ||
        type.element === compiler.boolClass ||
@@ skipping 341 matching lines @@
     if (result === null) {
       String fullConstructorName = cls.name.slowToString();
       if (constructorName !== const SourceString('')) {
         fullConstructorName = '$fullConstructorName'
                               '.${constructorName.slowToString()}';
       }
       return failOrReturnErroneousElement(cls, diagnosticNode,
                                           new SourceString(fullConstructorName),
                                           MessageKind.CANNOT_FIND_CONSTRUCTOR,
                                           [fullConstructorName]);
-    } else if (inConstContext &&
-               (result.modifiers == null || !result.modifiers.isConst())) {
+    } else if (inConstContext && !result.modifiers.isConst()) {
       error(diagnosticNode, MessageKind.CONSTRUCTOR_IS_NOT_CONST);
     }
     return result;
   }
 
   visitNewExpression(NewExpression node) {
     Node selector = node.send.selector;
     Element e = visit(selector);
     if (!Elements.isUnresolved(e) && e.kind === ElementKind.CLASS) {
       ClassElement cls = e;
@@ skipping 198 matching lines @@
 
   Element localLookup(SourceString name) => library.find(name);
   Element lookup(SourceString name) => localLookup(name);
   Element lexicalLookup(SourceString name) => localLookup(name);
 
   Element add(Element newElement) {
     throw "Cannot add an element in the top scope";
   }
   String toString() => '$element';
 }