[ES6 modules] FetchDescendants: skip fetch algorithm for descendants that are already ready

third_party/WebKit/Source/core/loader/modulescript/ModuleTreeLinker.cpp

Index: third_party/WebKit/Source/core/loader/modulescript/ModuleTreeLinker.cpp
diff --git a/third_party/WebKit/Source/core/loader/modulescript/ModuleTreeLinker.cpp b/third_party/WebKit/Source/core/loader/modulescript/ModuleTreeLinker.cpp
index baceb7eedd23f10e52a9ea52e7ffd19e0cf300e0..8a904b112ea8bc0ad2af6e157ad7480fd26cb69c 100644
--- a/third_party/WebKit/Source/core/loader/modulescript/ModuleTreeLinker.cpp
+++ b/third_party/WebKit/Source/core/loader/modulescript/ModuleTreeLinker.cpp
@@ -265,12 +265,23 @@ void ModuleTreeLinker::FetchDescendants() {
       return;
     }
 
-    // Step 4.3. Otherwise, if ancestor list does not contain url, append url to
-    // urls.
-
     // Modulator::resolveModuleSpecifier() impl must return either a valid url
     // or null.
     CHECK(url.IsValid());
+
+    // [nospec, optimization]
+    // If we can already resolve the descendant module script from
+    // the module map at this point, we can skip "internal module script
+    // graph fetching procedure" for the descendant.
+    if (ModuleScript* script = modulator_->GetFetchedModuleScript(url)) {
+      if (script->InstantiationState() ==
+          ModuleInstantiationState::kInstantiated)
+        continue;
+    }
+
+    // Step 4.3. Otherwise, if ancestor list does not contain url, append url to
+    // urls.
+
     if (!ancestor_list_with_url_.Contains(url))
       urls.push_back(url);
   }