Since we will only have builtin extensions for a while, the
more-complicated, watchdog-based extension duplication prevention has
been replaced with one that treats the extension URL as equivalent to
the extension ID. This is true only for built-in extensions, so once we
start supporting third-party extensions we will likely need to bring
back the watchdog (commit e4381b4693) and
the more-complex duplication prevention (commit
670e51d335).
The code here should be treated as a last resort: ideally the GUI should
check if a particular extension is loaded (maybe by calling the
Extension Manager's new `isExtensionLoaded` method) and avoid calling
`loadExtensionURL` if it is.
If an extension attempts to register with the same ID as another
extension which has already registered, the new registration is refused.
If the extension is in a worker and no other extension is successfully
registered by that worker, the watchdog system will terminate the
"empty" worker.
If an extension worker does not complete initialization in a reasonable
amount of time (currently 15 seconds) then the worker will be
terminated. A complete initialization includes registering an extension.
WeDo2 and Pen blocks have been converted to internal extensions, and can
now be loaded by giving `loadExtensionURL` the string 'pen' or 'wedo2'
instead of an actual URL.
The new `_registerInternalExtension` method on the extension manager
will register an extension object (an object with a `getInfo()` method)
with the extension system without sandboxing the object in a Worker.
When asked to load an extension, the Extension Manager starts up a new
Worker. That worker runs the message dispatch system as well as an
instance of the new `ExtensionWorker` class, which will load the desired
extension and register it with the extension system.
Extensions, placed in `./src/extensions/*.js`, are now processed by
Webpack as separate entry points and packed into an `extensions`
subdirectory in the output.
Still to do: query an extension's information, including the blocks it
provides, and register that information with the VM, GUI, etc.
