Using [ and ] in field names no longer works

[fedarko]

#3007 provides this example spec to show how to use field names that contain periods and brackets. However, this no longer seems to work: if you load this example in the Vega editor, nothing displays. (This happens both with the Vega-Lite v2 schema—which was specified in the example—and with the v3.2.1 schema.)

Also, trying to escape the source[yes] name by enclosing it in brackets as described here ([source[yes]]) fails as well—that gives you an error that says Access path missing open bracket: [source[yes]].

Here's a version of the schema where all I changed was removing the brackets (so now the y-field is sourceyes). As you can see, the plot now displays correctly (albeit with a single backslash in the source\.reco field name, which isn't ideal).