[web] Add 'nonce' prop to flutter.js loadEntrypoint (#137204)

## Description

This PR adds a `nonce` parameter to flutter.js' `loadEntrypoint` method.

When set, loadEntrypoint will add a `nonce` attribute to the `main.dart.js` script tag, which allows Flutter to run in environments slightly more restricted by CSP; those that don't add `'self'` as a valid source for `script-src`.

----

### CSP directive

After this change, the CSP directive for a Flutter Web index.html can be:

```
script-src 'nonce-YOUR_NONCE_VALUE' 'wasm-unsafe-eval';
font-src https://fonts.gstatic.com;
style-src 'nonce-YOUR_NONCE_VALUE';
```

When CSP is set via a `meta` tag (like in the test accompanying this change), and to use a service worker, the CSP needs an additional directive: [`worker-src 'self';`](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/worker-src)

When CSP set via response headers, the CSP that applies to `flutter_service_worker.js` is determined by its response headers. See **Web Workers API > [Content security policy](https://developer.mozilla.org/en-US/docs/Web/API/Web_Workers_API/Using_web_workers#content_security_policy)** in MDN.)

----

### Initialization

If the CSP is set to disallow `script-src 'self'`, a nonce needs to also be passed to `loadEntrypoint`:

```javascript
  _flutter.loader.loadEntrypoint({
    nonce: 'SOME_NONCE',
    onEntrypointLoaded: (engineInitializer) async {
      const appRunner = await engineInitializer.initializeEngine({
        nonce: 'SOME_NONCE',
      });
      appRunner.runApp();
    },
  });
```

(`nonce` shows twice for now, because the entrypoint loader script doesn't have direct access to the `initializeEngine` call.)

----

## Tests

* Added a smoke test to ensure an app configured as described above starts.

## Issues

* Fixes https://github.com/flutter/flutter/issues/126977

dev/bots/service_worker_test.dart

@@ -37,6 +37,8 @@ enum ServiceWorkerTestType {
   withFlutterJsEntrypointLoadedEvent,
   // Same as withFlutterJsEntrypointLoadedEvent, but with TrustedTypes enabled.
   withFlutterJsTrustedTypesOn,
+  // Same as withFlutterJsEntrypointLoadedEvent, but with nonce required.
+  withFlutterJsNonceOn,
   // Uses custom serviceWorkerVersion.
   withFlutterJsCustomServiceWorkerVersion,
   // Entrypoint generated by `flutter create`.
@@ -53,6 +55,7 @@ Future<void> main() async {
   await runWebServiceWorkerTest(headless: false, testType: ServiceWorkerTestType.withFlutterJsShort);
   await runWebServiceWorkerTest(headless: false, testType: ServiceWorkerTestType.withFlutterJsEntrypointLoadedEvent);
   await runWebServiceWorkerTest(headless: false, testType: ServiceWorkerTestType.withFlutterJsTrustedTypesOn);
+  await runWebServiceWorkerTest(headless: false, testType: ServiceWorkerTestType.withFlutterJsNonceOn);
   await runWebServiceWorkerTestWithCachingResources(headless: false, testType: ServiceWorkerTestType.withoutFlutterJs);
   await runWebServiceWorkerTestWithCachingResources(headless: false, testType: ServiceWorkerTestType.withFlutterJs);
   await runWebServiceWorkerTestWithCachingResources(headless: false, testType: ServiceWorkerTestType.withFlutterJsShort);
@@ -120,6 +123,8 @@ String _testTypeToIndexFile(ServiceWorkerTestType type) {
       indexFile = 'index_with_flutterjs_entrypoint_loaded.html';
     case ServiceWorkerTestType.withFlutterJsTrustedTypesOn:
       indexFile = 'index_with_flutterjs_el_tt_on.html';
+    case ServiceWorkerTestType.withFlutterJsNonceOn:
+      indexFile = 'index_with_flutterjs_el_nonce.html';
     case ServiceWorkerTestType.withFlutterJsCustomServiceWorkerVersion:
       indexFile = 'index_with_flutterjs_custom_sw_version.html';
     case ServiceWorkerTestType.generatedEntrypoint:

dev/bots/test.dart

@@ -1260,6 +1260,7 @@ Future<void> _runWebLongRunningTests() async {
     () => runWebServiceWorkerTest(headless: true, testType: ServiceWorkerTestType.withFlutterJsShort),
     () => runWebServiceWorkerTest(headless: true, testType: ServiceWorkerTestType.withFlutterJsEntrypointLoadedEvent),
     () => runWebServiceWorkerTest(headless: true, testType: ServiceWorkerTestType.withFlutterJsTrustedTypesOn),
+    () => runWebServiceWorkerTest(headless: true, testType: ServiceWorkerTestType.withFlutterJsNonceOn),
     () => runWebServiceWorkerTestWithCachingResources(headless: true, testType: ServiceWorkerTestType.withoutFlutterJs),
     () => runWebServiceWorkerTestWithCachingResources(headless: true, testType: ServiceWorkerTestType.withFlutterJs),
     () => runWebServiceWorkerTestWithCachingResources(headless: true, testType: ServiceWorkerTestType.withFlutterJsShort),

dev/integration_tests/web/web/index_with_flutterjs_el_nonce.html

@@ -0,0 +1,49 @@
+<!DOCTYPE HTML>
+<!-- Copyright 2014 The Flutter Authors. All rights reserved.
+Use of this source code is governed by a BSD-style license that can be
+found in the LICENSE file. -->
+<html>
+<head>
+  <meta charset="UTF-8">
+  <meta content="IE=Edge" http-equiv="X-UA-Compatible">
+
+  <title>Integration test. App load with flutter.js and onEntrypointLoaded API. nonce required.</title>
+
+  <!-- Enable a CSP that requires a nonce for script and style-src. -->
+  <meta http-equiv="Content-Security-Policy" content="script-src 'nonce-SOME_NONCE' 'wasm-unsafe-eval'; font-src https://fonts.gstatic.com; style-src 'nonce-SOME_NONCE'; worker-src 'self';">
+
+  <!-- iOS meta tags & icons -->
+  <meta name="apple-mobile-web-app-capable" content="yes">
+  <meta name="apple-mobile-web-app-status-bar-style" content="black">
+  <meta name="apple-mobile-web-app-title" content="Web Test">
+  <link rel="manifest" href="manifest.json">
+  <script nonce="SOME_NONCE">
+    // The value below is injected by flutter build, do not touch.
+    var serviceWorkerVersion = null;
+  </script>
+  <!-- This script adds the flutter initialization JS code -->
+  <script nonce="SOME_NONCE" src="flutter.js" defer></script>
+</head>
+<body>
+  <script nonce="SOME_NONCE">
+    window.addEventListener('load', function(ev) {
+      // Download main.dart.js
+      _flutter.loader.loadEntrypoint({
+        nonce: 'SOME_NONCE',
+        onEntrypointLoaded: onEntrypointLoaded,
+        serviceWorker: {
+          serviceWorkerVersion: serviceWorkerVersion,
+        }
+      });
+
+      // Once the entrypoint is ready, do things!
+      async function onEntrypointLoaded(engineInitializer) {
+        const appRunner = await engineInitializer.initializeEngine({
+          nonce: 'SOME_NONCE',
+        });
+        appRunner.runApp();
+      }
+    });
+  </script>
+</body>
+</html>

packages/flutter_tools/lib/src/web/file_generators/js/flutter.js

@@ -244,10 +244,10 @@ _flutter.loader = null;
      * Returns undefined when an `onEntrypointLoaded` callback is supplied in `options`.
      */
     async loadEntrypoint(options) {
-      const { entrypointUrl = `${baseUri}main.dart.js`, onEntrypointLoaded } =
+      const { entrypointUrl = `${baseUri}main.dart.js`, onEntrypointLoaded, nonce } =
         options || {};
 
-      return this._loadEntrypoint(entrypointUrl, onEntrypointLoaded);
+      return this._loadEntrypoint(entrypointUrl, onEntrypointLoaded, nonce);
     }
 
     /**
@@ -286,12 +286,12 @@ _flutter.loader = null;
      *                                is loaded, or undefined if `onEntrypointLoaded`
      *                                is a function.
      */
-    _loadEntrypoint(entrypointUrl, onEntrypointLoaded) {
+    _loadEntrypoint(entrypointUrl, onEntrypointLoaded, nonce) {
       const useCallback = typeof onEntrypointLoaded === "function";
 
       if (!this._scriptLoaded) {
         this._scriptLoaded = true;
-        const scriptTag = this._createScriptTag(entrypointUrl);
+        const scriptTag = this._createScriptTag(entrypointUrl, nonce);
         if (useCallback) {
           // Just inject the script tag, and return nothing; Flutter will call
           // `didCreateEngineInitializer` when it's done.
@@ -319,9 +319,12 @@ _flutter.loader = null;
      * @param {string} url
      * @returns {HTMLScriptElement}
      */
-    _createScriptTag(url) {
+    _createScriptTag(url, nonce) {
       const scriptTag = document.createElement("script");
       scriptTag.type = "application/javascript";
+      if (nonce) {
+        scriptTag.nonce = nonce;
+      }
       // Apply TrustedTypes validation, if available.
       let trustedUrl = url;
       if (this._ttPolicy != null) {