[dwds] callServiceExtension should check all extensions and return kMethodNotFound when extension not found

[srujzs]

Currently, callServiceExtension checks the extensionRPCs field in the VM service. This list is populated when DWDS starts and checks the extensions currently registered in the runtime. This list is prone to becoming stale, however, as services may be registered or removed through the runtime (via dart:developer) later.

Instead, we should seek to always check the runtime for the current registered extensions and then update this list whenever we do. It’s still prone to becoming stale, but it is less stale. Ideally, we would override extensionRPCs and fetch the current registered extensions, but that isn’t possible since it requires awaiting a future. We also can’t ignore this field altogether because it looks like clients, like Dart DevTools, uses this field. Note that extensionRPCs should be a subset of getExtensionRpcs since registerService isn't supported in DWDS:

webdev/dwds/lib/src/services/chrome_proxy_service.dart

Line 1088 in 8f146a1

Future<Success> registerService(String service, String alias) {

.

callServiceExtension should also return kMethodNotFound instead of its current JS evaluation error when an extension is not present.

[srujzs]

I'm a little unsure what the potential ramifications of this are, but considering we're just keeping information more updated and returning errors that correspond to the spec, hopefully it should be safe.

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This came up in #2585. The Flutter hot reload tests don't set up reassemble: https://github.com/flutter/flutter/blob/d14d2505f3aa748450ae11ca0996743c2eebf99a/packages/flutter_tools/test/integration.shard/test_data/hot_reload_test_common.dart#L1, so when using callServiceExtension, we return an unknown error due to a failed JS evaluation to call that service extension in the runtime. This then leads to test failures. Flutter tools ignores kMethodNotFound errors, but can and should not ignore unknown errors (like the failed evaluation).

So, the minimum needed to support these integration tests is for DWDS to return kMethodNotFound when the service extension is not found to align with the VM. Everything else in this PR is just to keep things less stale. I believe we can't really mock this service extension in the tests because we spin up a real Flutter process.

In general too, I've seen occasional logs pop up due to missing service extensions triggering errors when debugging random things for hot reload. It's hard to find a repro, but I suspect those should go away as well with this PR.