Use static final empty Modifier instead of null.

lib/compiler/implementation/compile_time_constants.dart

Index: lib/compiler/implementation/compile_time_constants.dart
diff --git a/lib/compiler/implementation/compile_time_constants.dart b/lib/compiler/implementation/compile_time_constants.dart
index f0358bdc90dd3699d9bde8dfd7219ab39c4fb041..1cd58bb7f9f054df53423613559973dcbd6e5332 100644
--- a/lib/compiler/implementation/compile_time_constants.dart
+++ b/lib/compiler/implementation/compile_time_constants.dart
@@ -398,12 +398,10 @@ class CompileTimeConstantEvaluator extends AbstractVisitor {
         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;
       }