Use static final empty Modifier instead of null.

lib/compiler/implementation/ssa/optimize.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 OptimizationPhase {
   String get name;
   void visitGraph(HGraph graph);
 }
 
 class SsaOptimizerTask extends CompilerTask {
@@ skipping 561 matching lines @@
     if (type === null) return null;
     return compiler.world.locateSingleField(type, selector);
   }
 
   HInstruction visitInvokeDynamicGetter(HInvokeDynamicGetter node) {
     Element field =
         findConcreteFieldForDynamicAccess(node.receiver, node.selector);
     if (field == null) return node;
 
     Modifiers modifiers = field.modifiers;
-    bool isFinalOrConst = false;
-    if (modifiers != null) {
-      isFinalOrConst = modifiers.isFinal() || modifiers.isConst();
-    }
+    bool isFinalOrConst = modifiers.isFinal() || modifiers.isConst();
     if (!compiler.resolverWorld.hasInvokedSetter(field, compiler)) {
       // If no setter is ever used for this field it is only initialized in the
       // initializer list.
       isFinalOrConst = true;
     }
     HFieldGet result = new HFieldGet(
         field, node.inputs[0], isAssignable: !isFinalOrConst);
     HType type = backend.optimisticFieldType(field);
     if (type != null) {
       result.guaranteedType = type;
@@ skipping 729 matching lines @@
     }
 
     // For other fields having setters in the generative constructor body, set
     // the type to UNKNOWN to avoid relying on the type set in the initializer
     // list.
     allSetters.forEach((Element element) {
       backend.registerFieldConstructor(element, HType.UNKNOWN);
     });
   }
 }