Ignore lints that will be triggered by ExternalDartReference changes (flutter/engine#53572)

https://dart-review.googlesource.com/c/sdk/+/370663 makes
ExternalDartReference generic. By doing so, it triggers two
cast_nullable_to_non_nullable lints that need to be silenced for it to
land. Once the above SDK changes land, this can be refactored to utilize
the generic and avoid the manual cast altogether.

engine/src/flutter/lib/web_ui/lib/src/engine/canvaskit/native_memory.dart

@@ -22,6 +22,7 @@ import 'package:ui/src/engine.dart';
 /// 6. We call `delete` on SkPaint.
 DomFinalizationRegistry _finalizationRegistry = DomFinalizationRegistry(
   (ExternalDartReference boxedUniq) {
+    // ignore: cast_nullable_to_non_nullable
     final UniqueRef<Object> uniq = boxedUniq.toDartObject as UniqueRef<Object>;
     uniq.collect();
   }.toJS

engine/src/flutter/lib/web_ui/lib/src/engine/skwasm/skwasm_impl/memory.dart

@@ -29,6 +29,7 @@ typedef DisposeFunction<T extends NativeType> = void Function(Pointer<T>);
 class SkwasmFinalizationRegistry<T extends NativeType> {
   SkwasmFinalizationRegistry(this.dispose)
     : registry = DomFinalizationRegistry(((ExternalDartReference address) =>
+      // ignore: cast_nullable_to_non_nullable
       dispose(Pointer<T>.fromAddress(address.toDartObject as int))
     ).toJS);