--dtd-uri (and server-provided storage) doesn't work with "dt serve" when embedded when SSE connection is disabled

[DanTup]

While testing in VS Code, I ran DevTools from source using the dart.customDevTools setting. In the log I can see it spawned the server with a --dtd-uri:

[10:37:14 AM] [DevTools] [Info] Spawning D:\Dev\Google\devtools\tool\bin\dt.bat with args ["serve","--machine","--allow-embedding","--dtd-uri","ws://127.0.0.1:65196/IoZPaHuLiGI="]
[10:37:14 AM] [DevTools] [Info] ..  in D:\Dev\Google\devtools
[10:37:14 AM] [DevTools] [Info] ..  with {"toolEnv":{"FLUTTER_HOST":"VSCode","PUB_ENVIRONMENT":"vscode.dart-code","FLUTTER_ROOT":"D:\\Dev\\Google\\Flutter\\Flutter main"},"envOverrides":{"LOCAL_DART_SDK":"D:\\Dev\\Google\\dart-sdk\\sdk","FLUTTER_ROOT":"D:\\Dev\\Google\\devtools\\tool\\flutter-sdk"}}
[10:37:14 AM] [DevTools] [Info]     PID: 13844

However, when I load something like the deeplinks page it doesn't know what my project roots are, and I see this in the console:

No DTD uri provided from the server during initialization.

It looks like in serve.dart this would be passed through to devtools_server/serve_local.dart in the DDS package in the SDK, so it's possible the actual issue is in the SDK, but here seemed like the right place to raise this.

[DanTup]

So this is actually a result of my change yesterday to not connect SSE when embedded.

There's code here that varies the storage based on whether there is an SSE connection:

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

Lines 40 to 45 in cfff32d

	final connection = await DevToolsServerConnection.connect();
	if (connection != null) {
	setGlobal(Storage, server.ServerConnectionStorage());
	} else {
	setGlobal(Storage, BrowserStorage());
	}

My guess is that this is because SSE used to be used for the storage (this was changed in #8284).

So this condition probably needs updating to detect whether the standard server API is available (without requiring SSE). But that probably raises a question about what this code should do:

devtools/packages/devtools_app/lib/src/shared/server/server.dart

Lines 40 to 41 in cfff32d

	bool get isDevToolsServerAvailable =>
	kIsWeb && storage is ServerConnectionStorage;

Does this mean "Is there an SSE server API available?" or is there a standard HTTP API server available? Perhaps it should be split into two booleans and then each place that uses it checked to see which it really cares about?

@kenzieschmoll LMK what you think (and whether I should revert my change for now until this is resolved).