code for super == not generated correctly

[jmesserly]

this affects package:source_span/src/file.dart

  bool operator ==(other) {
    if (other is! FileSpan) return super == other;
    return _start == other._start && _end == other._end &&
        sourceUrl == other.sourceUrl;
  }

The intent is to call the super['=='](other) but we generate this as dart.equals(super, other) which is not legal.

[jmesserly]

this is caught somehow transitively from package:test

[jmesserly]

https://codereview.chromium.org/1183453005