DevTools fails to load through proxies that don't support SSE

[DanTup]

I tried to launch DevTools in a cloud editor that doesn't support SSE (its connections will just hang indefinitely) and it failed to load (just leaving a white screen after printing the version number to the console).

The network tab looks like this:

I can't easily confirm, but I think it will be hanging on this await here:

devtools/packages/devtools_app/lib/main.dart

Line 28 in 43b2f5e

await preferences.init();

That init() call tries to use storage which will be a ServerConnectionStorage because DevToolsServerConnection.connect does not return null here:

devtools/packages/devtools_app/lib/src/config_specific/framework_initialize/_framework_initialize_web.dart

Lines 18 to 23 in f38f4ac

	// Here, we try and initialize the connection between the DevTools web app and
	// its local server. DevTools can be launched without the server however, so
	// establishing this connection is a best-effort.
	final connection = await DevToolsServerConnection.connect();
	if (connection != null) {
	setGlobal(Storage, ServerConnectionStorage(connection));

It doesn't return null, because in connect() the call to api/ping works fine (it's not SSE), which then unconditionally creates an SseClient:

devtools/packages/devtools_app/lib/src/server_api_client.dart

Line 28 in 86102bd

static Future<DevToolsServerConnection> connect() async {

If we want to support this, I think there are two options:

1. Support WebSockets instead of SSE for this API (DWDS already has some abstractions over SSE+WebSockets that could probably be taken)
2. Change this code to detect that SSE is not responsive and return null (this would effectively disable the server though, which may affect some functionality)

@devoncarew @jacob314 I don't know if this is currently worth pursuing, but thought it was worth capturing the issue somewhere perhaps to come back to.

[jacob314]

Fyi @grouma

[DanTup]

Another issue I believe is caused by SSE is not being able to open > 5 DevTools tabs, see #8298. If the fix for that issue is to switch from SSE to WS, that may also resolve this.