super and this in compile-time constants.

frog/leg/compiler.dart

Index: frog/leg/compiler.dart
diff --git a/frog/leg/compiler.dart b/frog/leg/compiler.dart
index 3cdedd5de1ab87a4834eae9ed0fee13f4fcb4843..8a0e6eedc5328de0df9939701def0e02f12cd8bd 100644
--- a/frog/leg/compiler.dart
+++ b/frog/leg/compiler.dart
@@ -385,7 +385,7 @@ class Compiler implements DiagnosticListener {
                               () => resolver.resolveSignature(element));
   }
 
-  Object compileVariable(VariableElement element) {
+  Constant compileVariable(VariableElement element) {
     return withCurrentElement(element, () {
       return constantHandler.compileVariable(element);
     });