diff --git a/pkg/compiler/lib/src/js_emitter/model.dart b/pkg/compiler/lib/src/js_emitter/model.dart
index 6bf9e2abdc6..e0a8fda68e2 100644
--- a/pkg/compiler/lib/src/js_emitter/model.dart
+++ b/pkg/compiler/lib/src/js_emitter/model.dart
@@ -282,6 +282,8 @@ class Class implements FieldContainer {
 
   int get superclassHolderIndex =>
       (superclass == null) ? 0 : superclass.holder.index;
+
+  String toString() => 'Class(${element.name})';
 }
 
 class MixinApplication extends Class {
diff --git a/pkg/compiler/lib/src/js_emitter/program_builder/collector.dart b/pkg/compiler/lib/src/js_emitter/program_builder/collector.dart
index c55ddec1413..5bc3f77db38 100644
--- a/pkg/compiler/lib/src/js_emitter/program_builder/collector.dart
+++ b/pkg/compiler/lib/src/js_emitter/program_builder/collector.dart
@@ -240,7 +240,13 @@ class Collector {
     // these are thought to not have been instantiated, so we neeed to be able
     // to identify them later and make sure we only emit "empty shells" without
     // fields, etc.
-    classesOnlyNeededForRti = _rtiNeededClasses.difference(neededClasses);
+    classesOnlyNeededForRti = new Set<ClassElement>();
+    for (ClassElement cls in _rtiNeededClasses) {
+      while (cls != null && !neededClasses.contains(cls)) {
+        if (!classesOnlyNeededForRti.add(cls)) break;
+        cls = cls.superclass;
+      }
+    }
 
     neededClasses.addAll(classesOnlyNeededForRti);
 
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 d18af44ffc2..d26428bc78b 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)]);
diff --git a/tests/language/language_dart2js.status b/tests/language/language_dart2js.status
index 368a346367e..7ab1da6f066 100644
--- a/tests/language/language_dart2js.status
+++ b/tests/language/language_dart2js.status
@@ -938,9 +938,3 @@ named_parameters_test/03: Crash # Issue 27394
 named_parameters_test/05: Crash # Issue 27394
 named_parameters_test/07: Crash # Issue 27394
 named_parameters_test/09: Crash # Issue 27394
-
-[ $compiler == dart2js && $runtime == d8 && $host_checked == false && $checked && $fast_startup == false]
-mixin_type_parameter6_test: RuntimeError #29441
-
-[ $compiler == dart2js && $runtime == d8 && $host_checked && $checked ]
-mixin_type_parameter6_test: Crash #29441