Compile-time error if mixing class has a factory

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 184772653046a9864fa61350381d2103eb188b76..67b3683c453ca9557eef2892b74e9aaf03e32e3a 100644
--- a/runtime/vm/class_finalizer.cc
+++ b/runtime/vm/class_finalizer.cc
@@ -2210,12 +2210,23 @@ void ClassFinalizer::ApplyMixinMembers(const Class& cls) {
   const intptr_t num_functions = functions.Length();
   for (intptr_t i = 0; i < num_functions; i++) {
     func ^= functions.At(i);
-    if (func.IsGenerativeConstructor()) {
+    if (func.IsFactory() || func.IsGenerativeConstructor()) {
       // A mixin class must not have explicit constructors.
       if (!func.IsImplicitConstructor()) {
-        ReportError(cls, cls.token_pos(),
-                    "mixin class '%s' must not have constructors\n",
-                    String::Handle(zone, mixin_cls.Name()).ToCString());
+        const char* ctr_kind = func.IsFactory() ? "factory" : "constructor";
+        const Script& script = Script::Handle(cls.script());
+        const Error& error = Error::Handle(
+            LanguageError::NewFormatted(Error::Handle(),
+                script, func.token_pos(), Report::AtLocation,
+                Report::kError, Heap::kNew,
+                "%s '%s' is illegal in mixin class %s",
+                ctr_kind,
+                String::Handle(func.PrettyName()).ToCString(),
+                String::Handle(zone, mixin_cls.Name()).ToCString()));
+
+        ReportErrors(error, cls, cls.token_pos(),
+                     "mixin class '%s' must not have constructors",
+                     String::Handle(zone, mixin_cls.Name()).ToCString());
       }
       continue;  // Skip the implicit constructor.
     }