dart2js: --fast-startup emitter could better encode case for tear-off with no optional arguments

[rakudrama]

There is some confusion in the encoding of tear-off closures between null (no optional arguments) and a function returning [] (takes zero optional positional arguments).

Example:

  installTearOff(J.JSArray.prototype, "get$contains", false, true, 1, function() {
    return [];
  }, ["call$1"], ["contains$1"], 10);

should be encoded as:

  installTearOff(J.JSArray.prototype, "get$contains", false, true, 1, null, ["call$1"], ["contains$1"], 10);

This would save ~0.1% (0% after gzip) but would parse faster since there would be no diet parsed functions.

[rakudrama]

Fixed by above commit.