Allows exception object to be null in simarm and simmips.

runtime/vm/simulator_arm.cc

Index: runtime/vm/simulator_arm.cc
===================================================================
--- runtime/vm/simulator_arm.cc	(revision 24343)
+++ runtime/vm/simulator_arm.cc	(working copy)
@@ -3073,7 +3073,7 @@
   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));