Use static final empty Modifier instead of null.

lib/compiler/implementation/compile_time_constants.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.
 
 /**
  * The [ConstantHandler] keeps track of compile-time constants,
  * initializations of global and static fields, and default values of
  * optional parameters.
  */
 class ConstantHandler extends CompilerTask {
@@ skipping 380 matching lines @@
     Element element = elements[send];
     if (send.isPropertyAccess) {
       if (Elements.isStaticOrTopLevelFunction(element)) {
         compiler.codegenWorld.staticFunctionsNeedingGetter.add(element);
         Constant constant = new FunctionConstant(element);
         compiler.constantHandler.registerCompileTimeConstant(constant);
         compiler.enqueuer.codegen.registerStaticUse(element);
         return constant;
       } else if (Elements.isStaticOrTopLevelField(element)) {
         Constant result;
-        if (element.modifiers !== null) {
-          if (element.modifiers.isConst()) {
-            result = compiler.compileConstant(element);
-          } else if (element.modifiers.isFinal() && !isEvaluatingConstant) {
-            result = compiler.compileVariable(element);
-          }
+        if (element.modifiers.isConst()) {
+          result = compiler.compileConstant(element);
+        } else if (element.modifiers.isFinal() && !isEvaluatingConstant) {
+          result = compiler.compileVariable(element);
         }
         if (result !== null) return result;
       }
       return signalNotCompileTimeConstant(send);
     } else if (send.isCall) {
       if (element === compiler.identicalFunction && send.argumentCount() == 2) {
         Constant left = evaluate(send.argumentsNode.nodes.head);
         Constant right = evaluate(send.argumentsNode.nodes.tail.head);
         Constant result = constantSystem.identity.fold(left, right);
         if (result !== null) return result;
@@ skipping 391 matching lines @@
       Constant fieldValue = fieldValues[field];
       if (fieldValue === null) {
         // Use the default value.
         fieldValue = compiler.compileConstant(field);
       }
       jsNewArguments.add(fieldValue);
     });
     return jsNewArguments;
   }
 }