Quick fix: nullify Isolate in background parsing slightly later.

src/background-parsing-task.cc

Index: src/background-parsing-task.cc
diff --git a/src/background-parsing-task.cc b/src/background-parsing-task.cc
index cbfaae5fe98eaa9ea39f9851b293bb59301890cf..99d9e9f74e0350d72a52b3ccf4137684a4e0c8d1 100644
--- a/src/background-parsing-task.cc
+++ b/src/background-parsing-task.cc
@@ -11,7 +11,7 @@ namespace internal {
 BackgroundParsingTask::BackgroundParsingTask(
     StreamedSource* source, ScriptCompiler::CompileOptions options,
     int stack_size, Isolate* isolate)
-    : source_(source), stack_size_(stack_size) {
+    : source_(source), stack_size_(stack_size), script_data_(nullptr) {
   // We don't set the context to the CompilationInfo yet, because the background
   // thread cannot do anything with it anyway. We set it just before compilation
   // on the foreground thread.
@@ -34,6 +34,14 @@ BackgroundParsingTask::BackgroundParsingTask(
   info->set_compile_options(options);
   // Parse eagerly with ignition since we will compile eagerly.
   info->set_allow_lazy_parsing(!(i::FLAG_ignition && i::FLAG_ignition_eager));
+
+  if (options == ScriptCompiler::kProduceParserCache ||
+      options == ScriptCompiler::kProduceCodeCache) {
+    source_->info->set_cached_data(&script_data_);
+  }
+  // Parser needs to stay alive for finalizing the parsing on the main
+  // thread.
+  source_->parser.reset(new Parser(source_->info.get()));
 }
 
 
@@ -42,33 +50,26 @@ void BackgroundParsingTask::Run() {
   DisallowHandleAllocation no_handles;
   DisallowHandleDereference no_deref;
 
-  ScriptData* script_data = NULL;
-  ScriptCompiler::CompileOptions options = source_->info->compile_options();
-  if (options == ScriptCompiler::kProduceParserCache ||
-      options == ScriptCompiler::kProduceCodeCache) {
-    source_->info->set_cached_data(&script_data);
-  }
+  // Reset the stack limit of the parser to reflect correctly that we're on a
+  // background thread.
+  uintptr_t stack_limit =
+      reinterpret_cast<uintptr_t>(&stack_limit) - stack_size_ * KB;
+  source_->parser->set_stack_limit(stack_limit);
 
   // Nullify the Isolate temporarily so that the background parser doesn't
   // accidentally use it.
   Isolate* isolate = source_->info->isolate();
   source_->info->set_isolate(nullptr);
 
-  uintptr_t stack_limit =
-      reinterpret_cast<uintptr_t>(&stack_limit) - stack_size_ * KB;
-
-  source_->info->set_stack_limit(stack_limit);
-  // Parser needs to stay alive for finalizing the parsing on the main
-  // thread. Passing &parse_info is OK because Parser doesn't store it.
-  source_->parser.reset(new Parser(source_->info.get()));
   source_->parser->ParseOnBackground(source_->info.get());
 
-  if (script_data != NULL) {
+  if (script_data_ != nullptr) {
     source_->cached_data.reset(new ScriptCompiler::CachedData(
-        script_data->data(), script_data->length(),
+        script_data_->data(), script_data_->length(),
         ScriptCompiler::CachedData::BufferOwned));
-    script_data->ReleaseDataOwnership();
-    delete script_data;
+    script_data_->ReleaseDataOwnership();
+    delete script_data_;
+    script_data_ = nullptr;
   }
   source_->info->set_isolate(isolate);
 }