Include superclasses of classes only needed for rti

pkg/compiler/lib/src/js_emitter/program_builder/program_builder.dart

Index: pkg/compiler/lib/src/js_emitter/program_builder/program_builder.dart
diff --git a/pkg/compiler/lib/src/js_emitter/program_builder/program_builder.dart b/pkg/compiler/lib/src/js_emitter/program_builder/program_builder.dart
index d18af44ffc27318c2536c8e064f54ce8191fce49..d26428bc78b5aba2c8b68bd483ec640db856e82f 100644
--- a/pkg/compiler/lib/src/js_emitter/program_builder/program_builder.dart
+++ b/pkg/compiler/lib/src/js_emitter/program_builder/program_builder.dart
@@ -209,7 +209,9 @@ class ProgramBuilder {
     _classes.forEach((ClassElement element, Class c) {
       if (element.superclass != null) {
         c.setSuperclass(_classes[element.superclass]);
-        assert(c.superclass != null);
+        assert(invariant(element, c.superclass != null,
+            message: "No Class for has been created for superclass "
+                "${element.superclass} of $c."));
       }
       if (c is MixinApplication) {
         c.setMixinClass(_classes[computeMixinClass(element)]);