[flutter_tools] make expando on vm service null safe to handle web stuff (#59624)

packages/flutter_tools/lib/src/vmservice.dart

@@ -444,9 +444,9 @@ class FlutterView {
 
 /// Flutter specific VM Service functionality.
 extension FlutterVmService on vm_service.VmService {
-  Uri get wsAddress => _wsAddressExpando[this];
+  Uri get wsAddress => this != null ? _wsAddressExpando[this] : null;
 
-  Uri get httpAddress => _httpAddressExpando[this];
+  Uri get httpAddress => this != null ? _httpAddressExpando[this] : null;
 
   /// Set the asset directory for the an attached Flutter view.
   Future<void> setAssetDirectory({

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

@@ -395,6 +395,13 @@ void main() {
     );
     expect(fakeVmServiceHost.hasRemainingExpectations, false);
   });
+
+  testWithoutContext('expandos are null safe', () {
+    vm_service.VmService vmService;
+
+    expect(vmService.httpAddress, null);
+    expect(vmService.wsAddress, null);
+  });
 }
 
 class MockDevice extends Mock implements Device {}