Print stack trace when Dart_SetReturnValue is passed UnwindError.

runtime/vm/dart_api_impl.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 "include/dart_api.h"
 #include "include/dart_mirrors_api.h"
 #include "include/dart_native_api.h"
 
 #include "platform/assert.h"
+#include "lib/stacktrace.h"
 #include "vm/class_finalizer.h"
 #include "vm/compiler.h"
 #include "vm/dart.h"
 #include "vm/dart_api_impl.h"
 #include "vm/dart_api_message.h"
 #include "vm/dart_api_state.h"
 #include "vm/dart_entry.h"
 #include "vm/debugger.h"
 #include "vm/debuginfo.h"
 #include "vm/exceptions.h"
@@ skipping 4765 matching lines @@
   }
   return Api::Success();
 }
 
 
 DART_EXPORT void Dart_SetReturnValue(Dart_NativeArguments args,
                                      Dart_Handle retval) {
   NativeArguments* arguments = reinterpret_cast<NativeArguments*>(args);
   ASSERT(arguments->thread()->isolate() == Isolate::Current());
   if ((retval != Api::Null()) && (!Api::IsInstance(retval))) {
+    // Print the current stack trace to make the problematic caller
+    // easier to find.
+    const Stacktrace& stacktrace = GetCurrentStacktrace(0);
+    OS::PrintErr("=== Current Trace:\n%s===\n", stacktrace.ToCString());
+
     const Object& ret_obj = Object::Handle(Api::UnwrapHandle(retval));
     FATAL1("Return value check failed: saw '%s' expected a dart Instance.",
            ret_obj.ToCString());
   }
   ASSERT(retval != 0);
   Api::SetReturnValue(arguments, retval);
 }
 
 
 DART_EXPORT void Dart_SetWeakHandleReturnValue(Dart_NativeArguments args,
@@ skipping 1162 matching lines @@
   return Api::Success();
 }
 #endif  // DART_PRECOMPILED_RUNTIME
 
 
 DART_EXPORT bool Dart_IsRunningPrecompiledCode() {
   return Dart::IsRunningPrecompiledCode();
 }
 
 }  // namespace dart