Make ClassFinalizer indifferent on whether we are generating a snapshot or not.

runtime/vm/object.cc

Index: runtime/vm/object.cc
===================================================================
--- runtime/vm/object.cc	(revision 11113)
+++ runtime/vm/object.cc	(working copy)
@@ -4367,7 +4367,9 @@
   }
   const Type& signature_type = Type::Handle(signature_class.SignatureType());
   if (!signature_type.IsFinalized()) {
-    ClassFinalizer::FinalizeType(
+    // No additional malformed types can be detected during finalization.
+    ClassFinalizer class_finalizer(ClassFinalizer::kNotGeneratingSnapshot);
+    class_finalizer.FinalizeType(
         signature_class, signature_type, ClassFinalizer::kCanonicalize);
   }
   ASSERT(closure_function.signature_class() == signature_class.raw());
@@ -6273,7 +6275,8 @@
 
 RawError* Library::Patch(const Script& script) const {
   ASSERT(script.kind() == RawScript::kPatchTag);
-  return Compiler::Compile(*this, script);
+  // Do not tolerate malformed types when patching libraries.
+  return Compiler::Compile(*this, script, ClassFinalizer::kGeneratingSnapshot);
 }