RFR: JDK-8259726: Use of HashSet leads to undefined order in test output

[Jonathan Gibbons]

Small cleanup in code, changing `HashSet` to `ArrayList`, with a corresponding beneficial impact on tests.

The irony is that this was originally done with the intent to change the element type from `String` to `HtmlId`, but having tried that, it seems better to localize the conversion from ids to class names, so I just cleaned up the expository comment a bit.

I rewrapped some of the long text block lines as well.

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Commit messages:
 - Merge remote-tracking branch 'upstream/master' into tabClasses
 - JDK-8259726: Use of HashSet leads to undefined order in test output

Changes: https://git.openjdk.java.net/jdk/pull/2086/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2086&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8259726
  Stats: 244 lines in 18 files changed: 34 ins; 2 del; 208 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2086.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2086/head:pull/2086

PR: https://git.openjdk.java.net/jdk/pull/2086