Refactor JSNumber.toDart and Object.toJS

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.

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;