[web] DRY up access to headers required for multi-threaded WebAssembly (#163555)

Author: kevmoo

packages/flutter_tools/lib/src/isolated/devfs_web.dart

@@ -41,6 +41,7 @@ import '../web/chrome.dart';
 import '../web/compile.dart';
 import '../web/memory_fs.dart';
 import '../web/module_metadata.dart';
+import '../web/web_constants.dart';
 import '../web_template.dart';
 
 typedef DwdsLauncher =
@@ -1315,11 +1316,8 @@ class ReleaseAssetServer {
           'Content-Type': mimeType,
           'Cross-Origin-Resource-Policy': 'cross-origin',
           'Access-Control-Allow-Origin': '*',
-          if (_needsCoopCoep &&
-              _fileSystem.path.extension(file.path) == '.html') ...<String, String>{
-            'Cross-Origin-Opener-Policy': 'same-origin',
-            'Cross-Origin-Embedder-Policy': 'credentialless',
-          },
+          if (_needsCoopCoep && _fileSystem.path.extension(file.path) == '.html')
+            ...kMultiThreadedHeaders,
         },
       );
     }
@@ -1329,10 +1327,7 @@ class ReleaseAssetServer {
       file.readAsBytesSync(),
       headers: <String, String>{
         'Content-Type': 'text/html',
-        if (_needsCoopCoep) ...<String, String>{
-          'Cross-Origin-Opener-Policy': 'same-origin',
-          'Cross-Origin-Embedder-Policy': 'credentialless',
-        },
+        if (_needsCoopCoep) ...kMultiThreadedHeaders,
       },
     );
   }

packages/flutter_tools/lib/src/test/flutter_web_platform.dart

@@ -32,6 +32,7 @@ import '../web/bootstrap.dart';
 import '../web/chrome.dart';
 import '../web/compile.dart';
 import '../web/memory_fs.dart';
+import '../web/web_constants.dart';
 import 'test_compiler.dart';
 import 'test_golden_comparator.dart';
 import 'test_time_recorder.dart';
@@ -57,10 +58,7 @@ shelf.Handler createDirectoryHandler(Directory directory, {required bool crossOr
       file.openRead(),
       headers: <String, String>{
         if (contentType != null) 'Content-Type': contentType,
-        if (needsCrossOriginIsolated) ...<String, String>{
-          'Cross-Origin-Opener-Policy': 'same-origin',
-          'Cross-Origin-Embedder-Policy': 'credentialless',
-        },
+        if (needsCrossOriginIsolated) ...kMultiThreadedHeaders,
       },
     );
   };
@@ -551,10 +549,7 @@ class FlutterWebPlatform extends PlatformPlugin {
       ''',
         headers: <String, String>{
           'Content-Type': 'text/html',
-          if (webRenderer == WebRendererMode.skwasm) ...<String, String>{
-            'Cross-Origin-Opener-Policy': 'same-origin',
-            'Cross-Origin-Embedder-Policy': 'credentialless',
-          },
+          if (webRenderer == WebRendererMode.skwasm) ...kMultiThreadedHeaders,
         },
       );
     }

packages/flutter_tools/lib/src/web/web_constants.dart

@@ -3,3 +3,12 @@
 // found in the LICENSE file.
 
 const String kWasmMoreInfo = 'See https://flutter.dev/to/wasm for more information.';
+
+/// Headers required to run Wasm-compiled applications with multi-threading.
+///
+/// See https://developer.chrome.com/blog/coep-credentialless-origin-trial
+/// for more information.
+const Map<String, String> kMultiThreadedHeaders = <String, String>{
+  'Cross-Origin-Opener-Policy': 'same-origin',
+  'Cross-Origin-Embedder-Policy': 'credentialless',
+};

packages/flutter_tools/test/general.shard/web/web_asset_server_test.dart

@@ -7,6 +7,7 @@ import 'package:flutter_tools/src/base/file_system.dart';
 import 'package:flutter_tools/src/base/io.dart';
 import 'package:flutter_tools/src/base/platform.dart';
 import 'package:flutter_tools/src/isolated/devfs_web.dart';
+import 'package:flutter_tools/src/web/web_constants.dart';
 import 'package:shelf/shelf.dart';
 
 import '../../src/common.dart';
@@ -236,8 +237,9 @@ void main() {
 
       expect(response.statusCode, HttpStatus.ok);
       final Map<String, String> headers = response.headers;
-      expect(headers['Cross-Origin-Opener-Policy'], 'same-origin');
-      expect(headers['Cross-Origin-Embedder-Policy'], 'credentialless');
+      for (final MapEntry<String, String> entry in kMultiThreadedHeaders.entries) {
+        expect(headers, containsPair(entry.key, entry.value));
+      }
     },
   );