implement null aware ops, fixes #249

lib/src/checker/checker.dart

Index: lib/src/checker/checker.dart
diff --git a/lib/src/checker/checker.dart b/lib/src/checker/checker.dart
index f537386aa40a6d9225628e97a22d182b88b87f76..dabada1ca39591de2a17fb164d1fafb07d9cba79 100644
--- a/lib/src/checker/checker.dart
+++ b/lib/src/checker/checker.dart
@@ -389,7 +389,8 @@ class CodeChecker extends RecursiveAstVisitor {
   @override
   void visitConstructorFieldInitializer(ConstructorFieldInitializer node) {
     var field = node.fieldName;
-    DartType staticType = rules.elementType(field.staticElement);
+    var element = field.staticElement;
+    DartType staticType = rules.elementType(element);
     checkAssignment(node.expression, staticType);
     node.visitChildren(this);
   }
@@ -792,6 +793,8 @@ class CodeChecker extends RecursiveAstVisitor {
           break;
         case TokenType.BANG_EQ:
           break;
+        case TokenType.QUESTION_QUESTION:
+          break;
         default:
           assert(false);
       }