Turn implicit casts in code generated by `flutter_platform.dart` into explicit casts. (#56290)

packages/flutter_tools/lib/src/test/flutter_platform.dart

@@ -200,9 +200,10 @@ void catchIsolateErrors() {
   errorPort.listen((dynamic message) {
     // Masquerade as an IsolateSpawnException because that's what this would
     // be if the error had been detected statically.
-    final IsolateSpawnException error = IsolateSpawnException(message[0]);
-    final Trace stackTrace =
-        message[1] == null ? Trace(const <Frame>[]) : Trace.parse(message[1]);
+    final IsolateSpawnException error = IsolateSpawnException(
+        message[0] as String);
+    final Trace stackTrace = message[1] == null ?
+        Trace(const <Frame>[]) : Trace.parse(message[1] as String);
     Zone.current.handleUncaughtError(error, stackTrace);
   });
 }
@@ -230,8 +231,7 @@ void main() {
   });
   WebSocket.connect(server).then((WebSocket socket) {
     socket.map((dynamic x) {
-      assert(x is String);
-      return json.decode(x);
+      return json.decode(x as String);
     }).pipe(channel.sink);
     socket.addStream(channel.stream.map(json.encode));
   });