Add Dart_PropagateError.

runtime/include/dart_api.h

Index: runtime/include/dart_api.h
===================================================================
--- runtime/include/dart_api.h	(revision 3804)
+++ runtime/include/dart_api.h	(working copy)
@@ -140,6 +140,30 @@
  */
 DART_EXPORT Dart_Handle Dart_Error(const char* format, ...);
 
+/**
+ * Propagates an error.
+ *
+ * It only makes sense to call this function when there are dart
+ * frames on the stack.  That is, this function should only be called
+ * in the C implementation of a native function which has been called
+ * from Dart code.  If this function is called in the top-level
+ * embedder code, it will return an error, as there is no way to
+ * further propagate the error.
+ *
+ * The provided handle must be an error handle.  (See Dart_IsError.)
+ *
+ * If the provided handle is an unhandled exception, this function
+ * will cause the unhandled exception to be rethrown.  Otherwise, the
+ * error will be propagated to the caller, discarding any active dart
+ * frames up to the next C frame.
+ *
+ * \param An error handle.
+ *
+ * \return On success, this function does not return.  On failure, an
+ *   error handle is returned.
+ */
+DART_EXPORT Dart_Handle Dart_PropagateError(Dart_Handle handle);
+
 // Internal routine used for reporting error handles.
 DART_EXPORT void _Dart_ReportErrorHandle(const char* file,
                                          int line,