Allows exception object to be null in simarm and simmips.

runtime/vm/simulator_arm.cc

 // Copyright (c) 2013, 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 <math.h>  // for isnan.
 #include <setjmp.h>
 #include <stdlib.h>
 
 #include "vm/globals.h"
 #if defined(TARGET_ARCH_ARM)
@@ skipping 3055 matching lines @@
   Isolate* isolate = Isolate::Current();
   while (isolate->top_resource() != NULL &&
          (reinterpret_cast<uword>(isolate->top_resource()) < native_sp)) {
     isolate->top_resource()->~StackResource();
   }
 
   // Unwind the C++ stack and continue simulation in the target frame.
   set_register(PC, static_cast<int32_t>(pc));
   set_register(SP, static_cast<int32_t>(sp));
   set_register(FP, static_cast<int32_t>(fp));
-  ASSERT(raw_exception != NULL);
+

[regis, 2013/06/24 17:30:47]

My concern is that we trash register kExceptionObjectReg when no exception is
reported, i.e. when raw_exception == NULL.
I suggest we do the same as for raw_stacktrace, i.e. we only set
kExceptionObjectReg when raw_exception != NULL.
We can then wait for Florian's cl and revisit. 

[zra, 2013/06/24 18:43:58]

raw_exception == NULL means that someone threw the Smi 0, so it is allowed.
Object::null() is not allowed here. raw_stacktrace can be anything so we pass it
through.

   set_register(kExceptionObjectReg, bit_cast<int32_t>(raw_exception));
   if (raw_stacktrace != NULL) {
     set_register(kStackTraceObjectReg, bit_cast<int32_t>(raw_stacktrace));
   }
   buf->Longjmp();
 }
 
 }  // namespace dart
 
 #endif  // !defined(HOST_ARCH_ARM)
 
 #endif  // defined TARGET_ARCH_ARM