Avoid allocating an exception object in the snapshot reader.

runtime/vm/object_store.h

Index: runtime/vm/object_store.h
===================================================================
--- runtime/vm/object_store.h	(revision 38227)
+++ runtime/vm/object_store.h	(working copy)
@@ -380,6 +380,13 @@
     out_of_memory_ = value.raw();
   }
 
+  RawUnhandledException* preallocated_unhandled_exception() const {
+    return preallocated_unhandled_exception_;
+  }
+  void set_preallocated_unhandled_exception(const UnhandledException& value) {
+    preallocated_unhandled_exception_ = value.raw();
+  }
+
   RawStacktrace* preallocated_stack_trace() const {
     return preallocated_stack_trace_;
   }
@@ -495,6 +502,7 @@
   RawContext* empty_context_;
   RawInstance* stack_overflow_;
   RawInstance* out_of_memory_;
+  RawUnhandledException* preallocated_unhandled_exception_;
   RawStacktrace* preallocated_stack_trace_;
   RawFunction* lookup_port_handler_;
   RawFunction* handle_message_function_;