Fix 15721: When checking for bad overrides we must eagerly finalize superclasses.

runtime/vm/class_finalizer.cc

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/class_finalizer.h"
 
 #include "vm/code_generator.h"
 #include "vm/flags.h"
 #include "vm/heap.h"
 #include "vm/isolate.h"
@@ skipping 1255 matching lines @@
   Error& error = Error::Handle();
   for (intptr_t i = 0; i < num_functions; i++) {
     function ^= array.At(i);
     ResolveAndFinalizeSignature(cls, function);
     name = function.name();
     if (FLAG_error_on_bad_override &&  // Report signature conflicts only.
         !function.is_static() && !function.IsConstructor()) {
       // A constructor cannot override anything.
       for (intptr_t i = 0; i < interfaces.Length(); i++) {
         super_class ^= interfaces.At(i);
+        // Finalize superclass since overrides check relies on all members
+        // of the superclass to be finalized.
+        FinalizeClass(super_class);
         overridden_function = super_class.LookupDynamicFunction(name);
         if (!overridden_function.IsNull() &&
             !function.HasCompatibleParametersWith(overridden_function,
                                                   &error)) {
           const String& class_name = String::Handle(cls.Name());
           const String& super_class_name = String::Handle(super_class.Name());
           const Script& script = Script::Handle(cls.script());
           ReportError(error, script, function.token_pos(),
                       "class '%s' overrides method '%s' of super class '%s' "
                       "with incompatible parameters",
@@ skipping 1514 matching lines @@
   expected_name ^= String::New("_offset");
   ASSERT(String::EqualsIgnoringPrivateKey(name, expected_name));
   field ^= fields_array.At(2);
   ASSERT(field.Offset() == TypedDataView::length_offset());
   name ^= field.name();
   ASSERT(name.Equals("length"));
 #endif
 }
 
 }  // namespace dart