Done by adding isKnownEmpty() to ResourceLoaderModule and overriding it to check for page existence in ResourceLoaderWikiModule. Needed to rearrange some code in OutputPage::makeResourceLoaderLink() to have the emptiness check and dropping of modules work properly. Also factored the page_touched check in ResourceLoaderWikiModule::getModifiedTime() out to a separate method (getTitleMtimes()) and moved in-object caching there as well, so getModifiedTime() and isKnownEmpty() share code and caching for their timestamp/existence checks.
This does not account for the case where e.g. a user has user CSS but no user JS: I had implemented this by checking for $context->getOnly() in getTitleMtimes(), but then realized it's not safe to do this in a function called by getModifiedTime(): it causes the timestamp list in the startup module to only take scripts in account for wiki modules, because the startup module has &only=scripts set
(Almost looks like it could all go into ResourceLoaderModule... But that uses a different version, seemingly, the only one. 3 other subclasses of ResourceLoaderModule implement the same version of getFlip as is moved into a parent class here... Seems daft to have a different version in the base abstract class... Minor oversight?)
Some documentation
* Specify page titles as strings instead of split NS/DBK, as suggested by Roan on CR. It seemed sensible to me.
* Pass a Title object to getContent() instead of a string, to avoid unnecessary object construction overhead
* "*" and "/" are valid title characters. Check module input for JS comment end tokens.
* Fixed inappropriate conversion to boolean, when checking result of getContent(). Presumably the idea was to omit empty sections and errors, so that's what I did. Maybe an informative error message would be better in the error case.
* Use LinkBatch for selecting multiple page rows instead of Database::makeWhereFrom2d().
* Fixed assignment expression.
TODO:
* Are there instances where we might want to restrict CSS as well as JS?
* Would a $wg config option and/or user preference and/or index.php GET parameter to limit inclusion be useful?
* Can we deprecate any of the existing $wg config options?
* What's going on with the duplicated code between OutputPage and SkinTemplate?
* For readability, rewrote ResourceLoaderFileModule::readScriptFiles() to use a loop instead of an excessively cute nested array_map() construction.
* In ResourceLoaderFileModule, check file_get_contents() result for errors. Golden rule of error checking: always check fopen(). Every call to fopen will fail some day for some user.
* In mediaWiki.loader.filter(): broke up a complex multi-line conditional into multiple commented if statements, for readability.
* Possessive "its" has no apostrophe.
* Modified Xml::encodeJsVar() to allow it to pass through JS expressions without encoding, using a special object.
* In ResourceLoader::makeModuleResponse(), renamed $messages to $messagesBlob to make it clear that it's JSON-encoded, not an array.
* Fixed MessageBlobStore to store {} for an empty message array instead of [].
* In ResourceLoader::makeMessageSetScript(), fixed call to non-existent function mediaWiki.msg.set.
* For security, changed the calling convention of makeMessageSetScript() and makeLoaderImplementScript() to require explicit object construction of XmlJsCode() before interpreting their input as JS code.
* Documented several ResourceLoader static functions.
* In ResourceLoaderWikiModule, for readability, reduced the indenting level by flipping some if blocks and adding continue statements.
* In makeCustomLoaderScript(), allow non-numeric $version. The only caller I can find is already sending a non-numeric $version, presumably it was broken. Luckily there aren't any loader scripts in existence, I had to make one to test it.
* wfGetDb -> wfGetDB
* Added an extra line break in the startup module output, for readability.
* In ResourceLoaderStartUpModule::getModuleRegistrations(), fixed another assignment expression
