Start supporting default classes.

dart/frog/leg/elements/elements.dart

Index: dart/frog/leg/elements/elements.dart
diff --git a/dart/frog/leg/elements/elements.dart b/dart/frog/leg/elements/elements.dart
index a040957d5cce3957361a4c547c3b2947d5bc0519..bc34765653118d4866ba7cf7bcedbbb4e27b9b61 100644
--- a/dart/frog/leg/elements/elements.dart
+++ b/dart/frog/leg/elements/elements.dart
@@ -491,6 +491,7 @@ class SynthesizedConstructorElement extends FunctionElement {
 class ClassElement extends ContainerElement {
   Type type;
   Type supertype;
+  Type defaultClass;
   Link<Element> members = const EmptyLink<Element>();
   Map<SourceString, Element> localMembers;
   Map<SourceString, Element> constructors;
@@ -612,6 +613,8 @@ class ClassElement extends ContainerElement {
     assert(isResolved);
     return supertype === null ? null : supertype.element;
   }
+
+  bool isInterface() => false;
 }
 
 class Elements {