Fix issue 2939, revert previous change that accepted null as excpetion objects:

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 45 matching lines @@
     frame = frames.NextFrame();
     ASSERT(frame != NULL);
   }
   ASSERT(frame->IsEntryFrame());
   *handler_pc = frame->pc();
   *handler_sp = frame->sp();
   *handler_fp = frame->fp();
 }
 
 
-static void ThrowExceptionHelper(const Instance& exception,
+static void ThrowExceptionHelper(const Instance& incoming_exception,
                                  const Instance& existing_stacktrace) {
+  Instance& exception = Instance::Handle(incoming_exception.raw());
+  if (exception.IsNull()) {
+    GrowableArray<const Object*> arguments;
+    exception ^= Exceptions::Create(Exceptions::kNullPointer, arguments);
+  }
   uword handler_pc = 0;
   uword handler_sp = 0;
   uword handler_fp = 0;
   GrowableArray<uword> stack_frame_pcs;
   bool handler_exists = FindExceptionHandler(&handler_pc,
                                              &handler_sp,
                                              &handler_fp,
                                              &stack_frame_pcs);
   // TODO(5411263): At some point we can optimize by figuring out if a
   // stack trace is needed based on whether the catch code specifies a
@@ skipping 273 matching lines @@
     case kIsolateSpawn:
       library = Library::IsolateLibrary();
       class_name = String::NewSymbol("IsolateSpawnException");
       break;
   }
 
   return DartLibraryCalls::ExceptionCreate(library, class_name, arguments);
 }
 
 }  // namespace dart