parsePlatformSettings method
Parses user-provided settings for a custom platform into a
plugin-defined format.
The settings come from a user's configuration file. The parsed output
will be passed to customizePlatform.
Subclasses should throw SourceSpanFormatExceptions if settings
contains invalid configuration. Unrecognized fields should be ignored if
possible.
Implementation
@override
ExecutableSettings parsePlatformSettings(YamlMap settings) {
  var map = Map.from(settings);
  map['headless'] = headless;
  var settings2 = YamlMap.wrap(map);
  return browserPlatform.parsePlatformSettings(settings2);
}