Fix assertion during throwing an exception.

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/cpu.h"
 #include "vm/dart_entry.h"
 #include "vm/flags.h"
 #include "vm/object.h"
@@ skipping 41 matching lines @@
   uword handler_sp = 0;
   uword handler_fp = 0;
   GrowableArray<uword> stack_frame_pcs;
   bool handler_exists = FindExceptionHandler(&handler_pc,
                                              &handler_sp,
                                              &handler_fp,
                                              &stack_frame_pcs);
   // 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.
-  ASSERT(stack_frame_pcs.length() > 0);  // At least one dart frame must exist.
   Stacktrace& stacktrace = Stacktrace::Handle();
-  if (existing_stacktrace.IsNull()) {
-    stacktrace = Stacktrace::New(stack_frame_pcs);
+  if (stack_frame_pcs.length() > 0) {
+    if (existing_stacktrace.IsNull()) {
+      stacktrace = Stacktrace::New(stack_frame_pcs);
+    } else {
+      stacktrace ^= existing_stacktrace.raw();
+      stacktrace.Append(stack_frame_pcs);
+    }
   } else {
     stacktrace ^= existing_stacktrace.raw();
-    stacktrace.Append(stack_frame_pcs);
   }
   if (FLAG_print_stack_trace_at_throw) {
     OS::Print("Exception '%s' thrown:\n", exception.ToCString());
     OS::Print("%s\n", stacktrace.ToCString());
   }
   if (handler_exists) {
     // Found a dart handler for the exception, jump to it.
     CPU::JumpToExceptionHandler(handler_pc,
                                 handler_sp,
                                 handler_fp,
@@ skipping 73 matching lines @@
       break;
     case kIllegalJSRegExp:
       class_name = String::NewSymbol("IllegalJSRegExpException");
       break;
   }
 
   return DartLibraryCalls::ExceptionCreate(class_name, arguments);
 }
 
 }  // namespace dart