[tool][web] Fix flutter.js in Safari 13 (#104761)

packages/flutter_tools/lib/src/web/file_generators/flutter_js.dart

@@ -27,14 +27,23 @@ _flutter.loader = null;
 (function() {
   "use strict";
   class FlutterLoader {
+    /**
+     * Creates a FlutterLoader, and initializes its instance methods.
+     */
+    constructor() {
       // TODO: Move the below methods to "#private" once supported by all the browsers
       // we support. In the meantime, we use the "revealing module" pattern.
 
       // Watchdog to prevent injecting the main entrypoint multiple times.
-    _scriptLoaded = null;
+      this._scriptLoaded = null;
 
       // Resolver for the pending promise returned by loadEntrypoint.
-    _didCreateEngineInitializerResolve = null;
+      this._didCreateEngineInitializerResolve = null;
+
+      // Called by Flutter web.
+      // Bound to `this` now, so "this" is preserved across JS <-> Flutter jumps.
+      this.didCreateEngineInitializer = this._didCreateEngineInitializer.bind(this);
+    }
 
     /**
      * Initializes the main.dart.js with/without serviceWorker.
@@ -51,19 +60,19 @@ _flutter.loader = null;
     }
 
     /**
-     * Resolves the promise created by loadEntrypoint. Called by Flutter.
-     * Needs to be weirdly bound like it is, so "this" is preserved across
-     * the JS <-> Flutter jumps.
+     * Resolves the promise created by loadEntrypoint.
+     * Called by Flutter through the public `didCreateEngineInitializer` method,
+     * which is bound to the correct instance of the FlutterLoader on the page.
      * @param {*} engineInitializer
      */
-    didCreateEngineInitializer = (function(engineInitializer) {
+    _didCreateEngineInitializer(engineInitializer) {
       if (typeof this._didCreateEngineInitializerResolve != "function") {
         console.warn("Do not call didCreateEngineInitializer by hand. Start with loadEntrypoint instead.");
       }
       this._didCreateEngineInitializerResolve(engineInitializer);
-      // Remove this method after it's done, so Flutter Web can hot restart.
+      // Remove the public method after it's done, so Flutter Web can hot restart.
       delete this.didCreateEngineInitializer;
-    }).bind(this);
+    }
 
     _loadEntrypoint(entrypointUrl) {
       if (!this._scriptLoaded) {
@@ -71,7 +80,11 @@ _flutter.loader = null;
           let scriptTag = document.createElement("script");
           scriptTag.src = entrypointUrl;
           scriptTag.type = "application/javascript";
-          this._didCreateEngineInitializerResolve = resolve; // Cache the resolve, so it can be called from Flutter.
+          // Cache the resolve, so it can be called from Flutter.
+          // Note: Flutter hot restart doesn't re-create this promise, so this
+          // can only be called once. Instead, we need to model this as a stream
+          // of `engineCreated` events coming from Flutter that are handled by JS.
+          this._didCreateEngineInitializerResolve = resolve;
           scriptTag.addEventListener("error", reject);
           document.body.append(scriptTag);
         });