Allow super type to be malformed (fixes issue 25426).

runtime/vm/class_finalizer.cc

Index: runtime/vm/class_finalizer.cc
diff --git a/runtime/vm/class_finalizer.cc b/runtime/vm/class_finalizer.cc
index 8ef3511c611dac6c57f9c0cc6ec2e40eced3fbdd..0361c038d19dc56fa355c63e954c3bd16608c6bb 100644
--- a/runtime/vm/class_finalizer.cc
+++ b/runtime/vm/class_finalizer.cc
@@ -2659,6 +2659,14 @@ RawType* ClassFinalizer::ResolveMixinAppType(
       AbstractType::Handle(zone, mixin_app_type.super_type());
   ResolveType(cls, mixin_super_type);
   ASSERT(mixin_super_type.HasResolvedTypeClass());  // Even if malformed.
+  if (mixin_super_type.IsMalformedOrMalbounded()) {
+    ReportError(Error::Handle(zone, mixin_super_type.error()));
+  }
+  if (mixin_super_type.IsDynamicType()) {
+    ReportError(cls, cls.token_pos(),
+                "class '%s' may not extend 'dynamic'",
+                String::Handle(zone, cls.Name()).ToCString());
+  }
   // The super type may have a BoundedType as type argument, but cannot be
   // a BoundedType itself.
   CollectTypeArguments(cls, Type::Cast(mixin_super_type), type_args);