Reland "Refactor JSNumber.toDart and Object.toJS" (#43363)

This reverts commit dce75ab.

This also makes some small changes to make onBenchmark a
JSExportedDartFunction instead of a JSBoxedDartObject. This is for
changes in flutter/flutter#129436 and to account
for the fact that flutter/packages provides an `allowInterop`'d
function. Benchmarks tests pass with this CL.

## Pre-launch Checklist

- [X] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [X] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [X] I read and followed the [Flutter Style Guide] and the [C++,
Objective-C, Java style guides].
- [X] I listed at least one issue that this PR fixes in the description
above.
- [ ] I added new tests to check the change I am making or feature I am
adding, or Hixie said the PR is test-exempt. See [testing the engine]
for instructions on writing and running engine tests.
- [X] I updated/added relevant documentation (doc comments with `///`).
- [X] I signed the [CLA].
- [ ] All existing and new tests are passing.

Author: srujzs

lib/web_ui/lib/src/engine/canvaskit/canvaskit_api.dart

@@ -205,10 +205,10 @@ Future<Uint8List> readChunked(HttpFetchPayload payload, int contentLength, WebOn
   int position = 0;
   int cumulativeBytesLoaded = 0;
   await payload.read<JSUint8Array1>((JSUint8Array1 chunk) {
-    cumulativeBytesLoaded += chunk.length.toDart.toInt();
+    cumulativeBytesLoaded += chunk.length.toDartInt;
     chunkCallback(cumulativeBytesLoaded, contentLength);
     result.set(chunk, position.toJS);
-    position += chunk.length.toDart.toInt();
+    position += chunk.length.toDartInt;
   });
   return result.toDart;
 }

lib/web_ui/lib/src/engine/canvaskit/image.dart

@@ -317,7 +317,7 @@ extension JsFlutterConfigurationExtension on JsFlutterConfiguration {
 
   @JS('canvasKitMaximumSurfaces')
   external JSNumber? get _canvasKitMaximumSurfaces;
-  double? get canvasKitMaximumSurfaces => _canvasKitMaximumSurfaces?.toDart;
+  double? get canvasKitMaximumSurfaces => _canvasKitMaximumSurfaces?.toDartDouble;
 
   @JS('debugShowSemanticsNodes')
   external JSBoolean? get _debugShowSemanticsNodes;