remove js_util

[jezell]

Fixes #48

[jezell]

Note that platform_detect is still a blocker on wasm builds, but that's the last one after this.

[theniceboy]

Incorrect type casting at https://github.com/jezell/dart-webrtc/blob/a6a18675e48fc7d6b89aa798a18283add17a6b60/lib/src/rtc_rtp_transceiver_impl.dart#L116

Should be

_jsTransceiver.setCodecPreferences(codecs
    .map((e) => e.toMap().jsify())
    .toList()
    .toJS as JSArray<web.RTCRtpCodec>);