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 24353)
+++ runtime/vm/simulator_arm.cc	(working copy)
@@ -3073,11 +3073,10 @@
   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);
+
+  ASSERT(raw_exception != Object::null());
   set_register(kExceptionObjectReg, bit_cast<int32_t>(raw_exception));
-  if (raw_stacktrace != NULL) {
-    set_register(kStackTraceObjectReg, bit_cast<int32_t>(raw_stacktrace));
-  }
+  set_register(kStackTraceObjectReg, bit_cast<int32_t>(raw_stacktrace));
   buf->Longjmp();
 }