Remove assert breaking pub after mixin fix.

runtime/vm/object.cc

Index: runtime/vm/object.cc
===================================================================
--- runtime/vm/object.cc	(revision 41446)
+++ runtime/vm/object.cc	(working copy)
@@ -5459,7 +5459,7 @@
 void Function::SetParameterTypeAt(
     intptr_t index, const AbstractType& value) const {
   ASSERT(!value.IsNull());
-  // Method extractor parameters are in the VM heap.
+  // Method extractor parameters are shared and are in the VM heap.
   ASSERT(kind() != RawFunction::kMethodExtractor);
   const Array& parameter_types = Array::Handle(raw_ptr()->parameter_types_);
   parameter_types.SetAt(index, value);