Deploy `@checked` (#6244)

This patch adds `@checked` everywhere is needed to remove the
`strong_mode_invalid_method_override` strong mode error.

packages/flutter_tools/lib/src/vmservice.dart

diff --git a/packages/flutter_tools/lib/src/vmservice.dart b/packages/flutter_tools/lib/src/vmservice.dart
index 8439244..690e0fa 100644
--- a/packages/flutter_tools/lib/src/vmservice.dart
+++ b/packages/flutter_tools/lib/src/vmservice.dart
@@ -868,15 +868,15 @@
   @override
   bool containsValue(dynamic v) => _map.containsValue(v);
   @override
-  bool containsKey(String k) => _map.containsKey(k);
+  bool containsKey(Object k) => _map.containsKey(k);
   @override
   void forEach(Function f) => _map.forEach(f);
   @override
   dynamic putIfAbsent(String key, Function ifAbsent) => _map.putIfAbsent(key, ifAbsent);
   @override
-  void remove(String key) => _map.remove(key);
+  void remove(Object key) => _map.remove(key);
   @override
-  dynamic operator [](String k) => _map[k];
+  dynamic operator [](Object k) => _map[k];
   @override
   void operator []=(String k, dynamic v) => _map[k] = v;
   @override