Fix minor bug which was causing throw of Null to not be recognized properly.

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/dart_api_impl.h"
 #include "vm/dart_entry.h"
 #include "vm/debugger.h"
 #include "vm/flags.h"
@@ skipping 346 matching lines @@
   bool handler_exists = false;
   if (use_preallocated_stacktrace) {
     stacktrace ^= isolate->object_store()->preallocated_stack_trace();
     PreallocatedStacktraceBuilder frame_builder(stacktrace);
     handler_exists = FindExceptionHandler(&handler_pc,
                                           &handler_sp,
                                           &handler_fp,
                                           &frame_builder);
   } else {
     const Field& stacktrace_field =
-        Field::Handle(LookupStacktraceField(incoming_exception));
+        Field::Handle(LookupStacktraceField(exception));
     bool full_stacktrace = !stacktrace_field.IsNull();
     RegularStacktraceBuilder frame_builder(full_stacktrace);
     handler_exists = FindExceptionHandler(&handler_pc,
                                           &handler_sp,
                                           &handler_fp,
                                           &frame_builder);
     // Create arrays for function, code and pc_offset triplet of each frame.
     const Array& func_array =
         Array::Handle(isolate, Array::MakeArray(frame_builder.func_list()));
     const Array& code_array =
@@ skipping 11 matching lines @@
       const Array& catch_code_array =
           Array::Handle(isolate,
                         Array::MakeArray(frame_builder.catch_code_list()));
       const Array& catch_pc_offset_array =
           Array::Handle(isolate,
                         Array::MakeArray(frame_builder.catch_pc_offset_list()));
       stacktrace = Stacktrace::New(func_array, code_array, pc_offset_array);
       stacktrace.SetCatchStacktrace(catch_func_array,
                                     catch_code_array,
                                     catch_pc_offset_array);
-      if (incoming_exception.GetField(stacktrace_field) == Object::null()) {
-        incoming_exception.SetField(stacktrace_field, stacktrace);
+      if (exception.GetField(stacktrace_field) == Object::null()) {
+        exception.SetField(stacktrace_field, stacktrace);
       }
     }
     // 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
     // stack trace object or there is a rethrow in the catch clause.
     if (pc_offset_array.Length() != 0) {
       if (existing_stacktrace.IsNull()) {
         stacktrace = Stacktrace::New(func_array, code_array, pc_offset_array);
       } else {
         stacktrace ^= existing_stacktrace.raw();
@@ skipping 290 matching lines @@
       break;
   }
 
   return DartLibraryCalls::InstanceCreate(library,
                                           *class_name,
                                           *constructor_name,
                                           arguments);
 }
 
 }  // namespace dart