Use static final empty Modifier instead of null.

pkg/dartdoc/lib/src/mirrors/dart2js_mirror.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.
 
 #library('mirrors.dart2js');
 
 #import('dart:io');
 #import('dart:uri');
 
 #import('../../../../../lib/compiler/compiler.dart', prefix: 'diagnostics');
@@ skipping 736 matching lines @@
       map[type.canonicalName] = type;
       link = link.tail;
     }
     return new ImmutableMapWrapper<Object, InterfaceMirror>(map);
   }
 
   bool get isClass => !_class.isInterface();
 
   bool get isInterface => _class.isInterface();
 
-  bool get isAbstract =>
-      _class.modifiers !== null && _class.modifiers.isAbstract();
+  bool get isAbstract => _class.modifiers.isAbstract();
 
   bool get isPrivate => _isPrivate(simpleName);
 
   bool get isDeclaration => true;
 
   List<TypeMirror> get typeArguments {
     throw new UnsupportedOperationException(
         'Declarations do not have type arguments');
   }
 
@@ skipping 477 matching lines @@
       int dollarPos = _name.indexOf('\$');
       if (dollarPos != -1) {
         _constructorName = _name.substring(dollarPos + 1);
         _name = _name.substring(0, dollarPos);
         // canonical name is TypeName.constructorName
         _canonicalName = '$_name.$_constructorName';
       } else {
         // canonical name is TypeName
         _canonicalName = _name;
       }
-      if (_function.modifiers !== null && _function.modifiers.isConst()) {
+      if (_function.modifiers.isConst()) {
         _kind = Dart2JsMethodKind.CONST;
       } else {
         _kind = Dart2JsMethodKind.CONSTRUCTOR;
       }
-    } else if (_function.modifiers !== null &&
-               _function.modifiers.isFactory()) {
+    } else if (_function.modifiers.isFactory()) {
       _kind = Dart2JsMethodKind.FACTORY;
       _constructorName = '';
       int dollarPos = _name.indexOf('\$');
       if (dollarPos != -1) {
         _constructorName = _name.substring(dollarPos+1);
         _name = _name.substring(0, dollarPos);
       }
       // canonical name is TypeName.constructorName
       _canonicalName = '$_name.$_constructorName';
     } else if (_name == 'negate') {
@@ skipping 30 matching lines @@
 
   bool get isConstructor
       => _kind == Dart2JsMethodKind.CONSTRUCTOR || isConst || isFactory;
 
   bool get isField => false;
 
   bool get isMethod => !isConstructor;
 
   bool get isPrivate => _isPrivate(simpleName);
 
-  bool get isStatic =>
-      _function.modifiers !== null && _function.modifiers.isStatic();
+  bool get isStatic => _function.modifiers.isStatic();
 
   List<ParameterMirror> get parameters {
     return _parametersFromFunctionSignature(system, this,
         _function.computeSignature(system.compiler));
   }
 
   TypeMirror get returnType => _convertTypeToTypeMirror(
       system, _function.computeSignature(system.compiler).returnType,
       system.compiler.types.dynamicType);
 
-  bool get isAbstract =>
-      _function.modifiers !== null && _function.modifiers.isAbstract();
+  bool get isAbstract => _function.modifiers.isAbstract();
 
   bool get isConst => _kind == Dart2JsMethodKind.CONST;
 
   bool get isFactory => _kind == Dart2JsMethodKind.FACTORY;
 
   String get constructorName => _constructorName;
 
   bool get isGetter => _kind == Dart2JsMethodKind.GETTER;
 
   bool get isSetter => _kind == Dart2JsMethodKind.SETTER;
@@ skipping 62 matching lines @@
     if (node !== null) {
       var span = system.compiler.spanFromNode(node, script.uri);
       return new Dart2JsLocation(script, span);
     } else {
       var span = system.compiler.spanFromElement(_variable);
       return new Dart2JsLocation(script, span);
     }
   }
 }