Print stack trace when Dart_SetReturnValue is passed UnwindError.

runtime/lib/stacktrace.h

Index: runtime/lib/stacktrace.h
diff --git a/runtime/lib/stacktrace.h b/runtime/lib/stacktrace.h
new file mode 100644
index 0000000000000000000000000000000000000000..e2bf6508743315159f717e9419f78487563e7648
--- /dev/null
+++ b/runtime/lib/stacktrace.h
@@ -0,0 +1,22 @@
+// Copyright (c) 2016, 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.
+
+#ifndef LIB_STACKTRACE_H_
+#define LIB_STACKTRACE_H_
+
+namespace dart {
+
+class Stacktrace;
+
+// Creates a Stacktrace object from the current stack.  Skips the
+// first skip_frames Dart frames.
+//
+// This function is exposed to provide stack trace printing in
+// assertion failures, etc.
+const Stacktrace& GetCurrentStacktrace(int skip_frames);
+
+
+}  // namespace dart
+
+#endif  // LIB_STACKTRACE_H_