Attempt to fix some web test flakiness. (#149828)

If the service is disposed when we make calls on it, we need to bubble up an exception that is understood by the callers.

This is another speculative fix for flutter/flutter#149238.

packages/flutter_tools/lib/src/vmservice.dart

@@ -989,6 +989,9 @@ class FlutterVmService {
         }
       }
       return await extensionAdded.future;
+    } on vm_service.RPCError {
+      // Translate this exception into something the outer layer understands
+      throw VmServiceDisappearedException();
     } finally {
       await isolateEvents.cancel();
       try {

packages/flutter_tools/test/general.shard/vmservice_test.dart

@@ -789,6 +789,17 @@ void main() {
         throwsA(isA<VmServiceDisappearedException>()),
       );
     });
+
+    testWithoutContext('throws when the service is disposed', () async {
+      final FakeVmServiceHost fakeVmServiceHost = FakeVmServiceHost(requests: <VmServiceExpectation>[]);
+
+      await fakeVmServiceHost.vmService.dispose();
+
+      expect(
+        () => fakeVmServiceHost.vmService.findExtensionIsolate(kExtensionName),
+        throwsA(isA<VmServiceDisappearedException>()),
+      );
+    });
   });
 
   testWithoutContext('Can process log events from the vm service', () {