Refactor JSNumber.toDart and Object.toJS (#43149)

JSNumber.toDart will now be two functions: toDartDouble and toDartInt.
Note that some code that looks like `toDart.toInt()` was kept as
`toDartDouble.toInt()` instead of `toDartInt`, since `toDartInt` throws
if the value is not an integer.

Object.toJS is now Object.toJSBox. An actual box is introduced on all
backends now, and is unwrapped in JSBoxedDartObject.toDart. There are
many usages of toJSAnyShallow that we should modify, but that's for a
later CL.

This is to help land this CL:
https://dart-review.googlesource.com/c/sdk/+/309082

https://dart-review.googlesource.com/c/sdk/+/309081 is the CL that added
the new methods.

## 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].
- [x] All existing and new tests are passing.

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

@@ -312,7 +312,7 @@ 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;