Add assert in the precompiler tree shaker to ensure that no redirecting

runtime/vm/precompiler.cc

Index: runtime/vm/precompiler.cc
diff --git a/runtime/vm/precompiler.cc b/runtime/vm/precompiler.cc
index d190d5a65be8e2cac93fe8df4410293dfcc3b460..67f975e0f5f8eb5db275815308e27b115864cdd7 100644
--- a/runtime/vm/precompiler.cc
+++ b/runtime/vm/precompiler.cc
@@ -1002,6 +1002,8 @@ void Precompiler::AddTypesOf(const Class& cls) {
 
 void Precompiler::AddTypesOf(const Function& function) {
   if (function.IsNull()) return;
+  // We don't expect to see a reference to a redicting factory.
+  ASSERT(!function.IsRedirectingFactory());
   if (functions_to_retain_.Lookup(&function) != NULL) return;
   functions_to_retain_.Insert(&Function::ZoneHandle(Z, function.raw()));