Fix rethrow by allowing exception object to be Null Object.

runtime/vm/exceptions.cc

 // Copyright (c) 2011, 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/exceptions.h"
 
 #include "vm/cpu.h"
 #include "vm/dart_entry.h"
 #include "vm/flags.h"
 #include "vm/object.h"
@@ skipping 101 matching lines @@
     CPU::JumpToErrorHandler(handler_pc,
                             handler_sp,
                             handler_fp,
                             unhandled_exception);
   }
   UNREACHABLE();
 }
 
 
 void Exceptions::Throw(const Instance& exception) {
+  // Null object is a valid exception object.
   ThrowExceptionHelper(exception, Instance::Handle());
 }
 
 
 void Exceptions::ReThrow(const Instance& exception,
                          const Instance& stacktrace) {
-  ASSERT(!exception.IsNull());
+  // Null object is a valid exception object.
   ThrowExceptionHelper(exception, stacktrace);
 }
 
 
 void Exceptions::PropagateError(const Object& obj) {
   ASSERT(Isolate::Current()->top_exit_frame_info() != 0);
   Error& error = Error::Handle();
   error ^= obj.raw();
   if (error.IsUnhandledException()) {
     // If the error object represents an unhandled exception, then
@@ skipping 74 matching lines @@
       break;
     case kIllegalJSRegExp:
       class_name = String::NewSymbol("IllegalJSRegExpException");
       break;
   }
 
   return DartLibraryCalls::ExceptionCreate(class_name, arguments);
 }
 
 }  // namespace dart