VM: Don't overwrite stacktrace when rethrowing StackOverflow.

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 "platform/address_sanitizer.h"
 
 #include "vm/dart_api_impl.h"
 #include "vm/dart_entry.h"
@@ skipping 403 matching lines @@
       // No Dart frame.
       ASSERT(incoming_exception.raw() ==
              isolate->object_store()->out_of_memory());
       const UnhandledException& error = UnhandledException::Handle(
           zone, isolate->object_store()->preallocated_unhandled_exception());
       thread->long_jump_base()->Jump(1, error);
       UNREACHABLE();
     }
     stacktrace ^= isolate->object_store()->preallocated_stack_trace();
     PreallocatedStackTraceBuilder frame_builder(stacktrace);
-    if (handler_needs_stacktrace) {
+    ASSERT(existing_stacktrace.IsNull() ||
+           (existing_stacktrace.raw() == stacktrace.raw()));
+    ASSERT(existing_stacktrace.IsNull() || is_rethrow);
+    if (handler_needs_stacktrace && existing_stacktrace.IsNull()) {
       BuildStackTrace(&frame_builder);
     }
   } else {
     if (!existing_stacktrace.IsNull()) {
       // If we have an existing stack trace then this better be a rethrow. The
       // reverse is not necessarily true (e.g. Dart_PropagateError can cause
       // a rethrow being called without an existing stacktrace.)
       ASSERT(is_rethrow);
       stacktrace = existing_stacktrace.raw();
     } else {
@@ skipping 375 matching lines @@
       class_name = &Symbols::_CompileTimeError();
       break;
   }
 
   return DartLibraryCalls::InstanceCreate(library, *class_name,
                                           *constructor_name, arguments);
 }
 
 
 }  // namespace dart