Implement parsing and resolving of type-variables.

frog/leg/ssa/codegen.dart

Index: frog/leg/ssa/codegen.dart
diff --git a/frog/leg/ssa/codegen.dart b/frog/leg/ssa/codegen.dart
index 7c15342fd83f977caa2b01215cdce01b557b2cd7..5c0d1bf97c074a7fc2a67f1c5a401469cb023f9d 100644
--- a/frog/leg/ssa/codegen.dart
+++ b/frog/leg/ssa/codegen.dart
@@ -970,7 +970,10 @@ class SsaCodeGenerator implements HVisitor {
   }
 
   void visitIs(HIs node) {
-    Element element = node.typeExpression;
+    ClassElement element = node.typeExpression;
+    if (node.typeExpression.kind === ElementKind.TYPE_VARIABLE) {
+      compiler.unimplemented("visitIs for type variables");
+    }
     compiler.registerIsCheck(element);
     LibraryElement coreLibrary = compiler.coreLibrary;
     ClassElement objectClass = coreLibrary.find(const SourceString('Object'));