Fix rethrow by allowing exception object to be Null Object.

runtime/vm/exceptions.cc

Index: runtime/vm/exceptions.cc
===================================================================
--- runtime/vm/exceptions.cc	(revision 4300)
+++ runtime/vm/exceptions.cc	(working copy)
@@ -119,13 +119,14 @@
 
 
 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);
 }