Support dart2wasm

[leecommamichael]

To reproduce:

• have the dart2wasm compiler
• import this package into a main.dart with void main
• run dart compile wasm main.dart Output:

../../../AppData/Local/Pub/Cache/hosted/pub.dev/pointycastle-3.7.4/lib/src/platform_check/node_crypto.dart:5:1: Error: JS interop library 'package:js/js.dart' can't be imported when compiling to Wasm. Try using 'dart:js_interop' or 'dart:js_interop_unsafe' instead. import 'package:js/js.dart'; ^ ../../../AppData/Local/Pub/Cache/hosted/pub.dev/pointycastle-3.7.4/lib/src/platform_check/web.dart:5:1: Error: JS interop library 'package:js/js.dart' can't be imported when compiling to Wasm. Try using 'dart:js_interop' or 'dart:js_interop_unsafe' instead. import 'package:js/js.dart'; ^

This library impacts the popular package:image. I've filed a matching report with them here.