Add a switch to use WebSockets for web debug proxy (#54083)

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

diff --git a/packages/flutter_tools/lib/src/build_runner/devfs_web.dart b/packages/flutter_tools/lib/src/build_runner/devfs_web.dart
index 67efb90..57341bc 100644
--- a/packages/flutter_tools/lib/src/build_runner/devfs_web.dart
+++ b/packages/flutter_tools/lib/src/build_runner/devfs_web.dart
@@ -131,6 +131,7 @@
     String hostname,
     int port,
     UrlTunneller urlTunneller,
+    bool useSseForDebugProxy,
     BuildMode buildMode,
     bool enableDwds,
     Uri entrypoint,
@@ -210,6 +211,7 @@
         },
         urlEncoder: urlTunneller,
         enableDebugging: true,
+        useSseForDebugProxy: useSseForDebugProxy,
         serveDevTools: false,
         logWriter: (Level logLevel, String message) => globals.printTrace(message),
         loadStrategy: RequireStrategy(
@@ -550,6 +552,7 @@
     @required this.port,
     @required this.packagesFilePath,
     @required this.urlTunneller,
+    @required this.useSseForDebugProxy,
     @required this.buildMode,
     @required this.enableDwds,
     @required this.entrypoint,
@@ -562,6 +565,7 @@
   final int port;
   final String packagesFilePath;
   final UrlTunneller urlTunneller;
+  final bool useSseForDebugProxy;
   final BuildMode buildMode;
   final bool enableDwds;
   final bool testMode;
@@ -623,6 +627,7 @@
       hostname,
       port,
       urlTunneller,
+      useSseForDebugProxy,
       buildMode,
       enableDwds,
       entrypoint,

packages/flutter_tools/lib/src/build_runner/resident_web_runner.dart

diff --git a/packages/flutter_tools/lib/src/build_runner/resident_web_runner.dart b/packages/flutter_tools/lib/src/build_runner/resident_web_runner.dart
index 6131a26..777a67e 100644
--- a/packages/flutter_tools/lib/src/build_runner/resident_web_runner.dart
+++ b/packages/flutter_tools/lib/src/build_runner/resident_web_runner.dart
@@ -423,6 +423,7 @@
           port: hostPort,
           packagesFilePath: packagesFilePath,
           urlTunneller: urlTunneller,
+          useSseForDebugProxy: debuggingOptions.webUseSseForDebugProxy,
           buildMode: debuggingOptions.buildInfo.mode,
           enableDwds: _enableDwds,
           entrypoint: globals.fs.file(target).uri,

packages/flutter_tools/lib/src/commands/run.dart

diff --git a/packages/flutter_tools/lib/src/commands/run.dart b/packages/flutter_tools/lib/src/commands/run.dart
index 5d5a4b5..56b0af3 100644
--- a/packages/flutter_tools/lib/src/commands/run.dart
+++ b/packages/flutter_tools/lib/src/commands/run.dart
@@ -351,6 +351,7 @@
         initializePlatform: boolArg('web-initialize-platform'),
         hostname: featureFlags.isWebEnabled ? stringArg('web-hostname') : '',
         port: featureFlags.isWebEnabled ? stringArg('web-port') : '',
+        webUseSseForDebugProxy: featureFlags.isWebEnabled && stringArg('web-server-debug-protocol') == 'sse',
         webEnableExposeUrl: featureFlags.isWebEnabled && boolArg('web-allow-expose-url'),
         webRunHeadless: featureFlags.isWebEnabled && boolArg('web-run-headless'),
         webBrowserDebugPort: browserDebugPort,
@@ -376,6 +377,7 @@
         initializePlatform: boolArg('web-initialize-platform'),
         hostname: featureFlags.isWebEnabled ? stringArg('web-hostname') : '',
         port: featureFlags.isWebEnabled ? stringArg('web-port') : '',
+        webUseSseForDebugProxy: featureFlags.isWebEnabled && stringArg('web-server-debug-protocol') == 'sse',
         webEnableExposeUrl: featureFlags.isWebEnabled && boolArg('web-allow-expose-url'),
         webRunHeadless: featureFlags.isWebEnabled && boolArg('web-run-headless'),
         webBrowserDebugPort: browserDebugPort,

packages/flutter_tools/lib/src/device.dart

diff --git a/packages/flutter_tools/lib/src/device.dart b/packages/flutter_tools/lib/src/device.dart
index 85309ff..b5a54fb 100644
--- a/packages/flutter_tools/lib/src/device.dart
+++ b/packages/flutter_tools/lib/src/device.dart
@@ -585,6 +585,7 @@
     this.hostname,
     this.port,
     this.webEnableExposeUrl,
+    this.webUseSseForDebugProxy = true,
     this.webRunHeadless = false,
     this.webBrowserDebugPort,
     this.vmserviceOutFile,
@@ -596,6 +597,7 @@
       this.port,
       this.hostname,
       this.webEnableExposeUrl,
+      this.webUseSseForDebugProxy = true,
       this.webRunHeadless = false,
       this.webBrowserDebugPort,
       this.cacheSkSL = false,
@@ -640,6 +642,7 @@
   final String port;
   final String hostname;
   final bool webEnableExposeUrl;
+  final bool webUseSseForDebugProxy;
 
   /// Whether to run the browser in headless mode.
   ///

packages/flutter_tools/lib/src/runner/flutter_command.dart

diff --git a/packages/flutter_tools/lib/src/runner/flutter_command.dart b/packages/flutter_tools/lib/src/runner/flutter_command.dart
index a6d7d2b..f90d8ef 100644
--- a/packages/flutter_tools/lib/src/runner/flutter_command.dart
+++ b/packages/flutter_tools/lib/src/runner/flutter_command.dart
@@ -168,6 +168,15 @@
         'will select a random open port on the host.',
       hide: hide,
     );
+    argParser.addOption('web-server-debug-protocol',
+      allowed: <String>['sse', 'ws'],
+      defaultsTo: 'sse',
+      help: 'The protocol (SSE or WebSockets) to use for the debug service proxy '
+      'when using the Web Server device and Dart Debugger extension. '
+      'This is useful for editors/debug adapters that do not support debugging '
+      'over SSE (the default protocol for Web Server/Dart Debugger extension).',
+      hide: hide,
+    );
     argParser.addFlag('web-allow-expose-url',
       defaultsTo: false,
       help: 'Enables daemon-to-editor requests (app.exposeUrl) for exposing URLs '