Make isEmpty a getter.

lib/compiler/implementation/tree_validator.dart

Index: lib/compiler/implementation/tree_validator.dart
diff --git a/lib/compiler/implementation/tree_validator.dart b/lib/compiler/implementation/tree_validator.dart
index bef2891dceef1d5c6c85593cc8cf74e3c0854b07..5b8a9b06259960e986a35b5423ec6804869b7a79 100644
--- a/lib/compiler/implementation/tree_validator.dart
+++ b/lib/compiler/implementation/tree_validator.dart
@@ -19,7 +19,7 @@ class TreeValidatorTask extends CompilerTask {
     final validator = new ValidatorVisitor(report);
     tree.accept(new TraversingVisitor(validator));
 
-    return errors.isEmpty();
+    return errors.isEmpty;
   }
 }
 
@@ -44,12 +44,12 @@ class ValidatorVisitor extends Visitor {
     if (identical(name, '++') || identical(name, '--')) {
       expect(node, node.assignmentOperator is Operator);
       if (node.isIndex) {
-        expect(node.arguments.tail.head, node.arguments.tail.isEmpty());
+        expect(node.arguments.tail.head, node.arguments.tail.isEmpty);
       } else {
-        expect(node.arguments.head, node.arguments.isEmpty());
+        expect(node.arguments.head, node.arguments.isEmpty);
       }
     } else {
-      expect(node, !node.arguments.isEmpty());
+      expect(node, !node.arguments.isEmpty);
     }
   }