Reserve the default context for V8 internals (#782)

V8 uses the default context for internal things
https://chromium.googlesource.com/v8/v8/+/refs/heads/main/src/inspector/v8-inspector-impl.cc#356

With this PR, deno_core will create a new default context during
snapshot and add deno_core's context separately (as the last context of
the snapshot)

Ref denoland/deno#24204

Fixes denoland/deno#24196

core/runtime/jsruntime.rs

@@ -780,6 +780,7 @@ impl JsRuntime {
  // SAFETY: We attach external_refs to IsolateAllocations which will live as long as the isolate
  let external_refs_static = unsafe { std::mem::transmute(external_refs) };
 
+ let has_snapshot = maybe_startup_snapshot.is_some();
  let mut isolate = setup::create_isolate(
  will_snapshot,
  options.create_params.take(),
@@ -836,6 +837,7 @@ impl JsRuntime {
  scope,
  &global_template_middleware,
  &global_object_middlewares,
+ has_snapshot,
  );
 
  // Get module map data from the snapshot
@@ -1882,15 +1884,23 @@ fn create_context<'a>(
  scope: &mut v8::HandleScope<'a, ()>,
  global_template_middlewares: &[GlobalTemplateMiddlewareFn],
  global_object_middlewares: &[GlobalObjectMiddlewareFn],
+ has_snapshot: bool,
 ) -> v8::Local<'a, v8::Context> {
- // Set up the global object template and create context from it.
- let mut global_object_template = v8::ObjectTemplate::new(scope);
- for middleware in global_template_middlewares {
- global_object_template = middleware(scope, global_object_template);
- }
+ let context = if has_snapshot {
+ // Try to load the 1st index first, embedder may have used 0th for something else (like node:vm).
+ v8::Context::from_snapshot(scope, 1)
+ .unwrap_or_else(|| v8::Context::from_snapshot(scope, 0).unwrap())
+ } else {
+ // Set up the global object template and create context from it.
+ let mut global_object_template = v8::ObjectTemplate::new(scope);
+ for middleware in global_template_middlewares {
+ global_object_template = middleware(scope, global_object_template);
+ }
+
+ global_object_template.set_internal_field_count(2);
+ v8::Context::new_from_template(scope, global_object_template)
+ };
 
- global_object_template.set_internal_field_count(2);
- let context = v8::Context::new_from_template(scope, global_object_template);
  let scope = &mut v8::ContextScope::new(scope, context);
 
  // Get the global wrapper object from the context, get the real inner
@@ -1951,8 +1961,11 @@ impl JsRuntimeForSnapshot {
  // Set the context to be snapshot's default context
  {
  let mut scope = realm.handle_scope(self.v8_isolate());
+ let default_context = v8::Context::new(&mut scope);
+ scope.set_default_context(default_context);
+
  let local_context = v8::Local::new(&mut scope, realm.context());
- scope.set_default_context(local_context);
+ scope.add_context(local_context);
  }
 
  // Borrow the source maps during the snapshot to avoid copies