Add a command line flag to disable streaming.

src/api.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/api.h"
 
 #include <string.h>  // For memcpy, strlen.
 #ifdef V8_USE_ADDRESS_SANITIZER
 #include <sanitizer/asan_interface.h>
 #endif  // V8_USE_ADDRESS_SANITIZER
@@ skipping 2197 matching lines @@
     size_t context_extension_count, Local<Object> context_extensions[]) {
   RETURN_TO_LOCAL_UNCHECKED(
       CompileFunctionInContext(v8_context, source, arguments_count, arguments,
                                context_extension_count, context_extensions),
       Function);
 }
 
 
 ScriptCompiler::ScriptStreamingTask* ScriptCompiler::StartStreamingScript(
     Isolate* v8_isolate, StreamedSource* source, CompileOptions options) {
+  if (!i::FLAG_script_streaming) {
+    return nullptr;
+  }
   i::Isolate* isolate = reinterpret_cast<i::Isolate*>(v8_isolate);
   return new i::BackgroundParsingTask(source->impl(), options,
                                       i::FLAG_stack_size, isolate);
 }
 
 
 MaybeLocal<Script> ScriptCompiler::Compile(Local<Context> context,
                                            StreamedSource* v8_source,
                                            Local<String> full_source_string,
                                            const ScriptOrigin& origin) {
@@ skipping 7136 matching lines @@
   Address callback_address =
       reinterpret_cast<Address>(reinterpret_cast<intptr_t>(callback));
   VMState<EXTERNAL> state(isolate);
   ExternalCallbackScope call_scope(isolate, callback_address);
   callback(info);
 }
 
 
 }  // namespace internal
 }  // namespace v8