"is not a type" error in /usr/share/unity8/Components/PhysicalKeysMapper.qml throws phone into UI restart loop

This morning, after upgrading "Ubuntu" on my phone using the SystemSettings->Updates icon, I had the displeasure of seeing the phone stuck in boot. The big ubuntu circle with dots underneath would show, flicker briefly, and continue, without ever prompting me for my SIM pincode, or giving me access to the UI.

Fortunately, the phone would still connect to my Wifi network, so I could connect to it with ssh to investigate.

It turned out that unity8 crashed with the following error:

file:///usr/share/unity8//OrientedShell.qml:181:5: Type Shell unavailable
         Shell {
         ^
file:///usr/share/unity8//Shell.qml:160:5: Type PhysicalKeysMapper unavailable
         PhysicalKeysMapper {
         ^
file:///usr/share/unity8//Components/PhysicalKeysMapper.qml:70:5: InputEventGenerator is not a type
         InputEventGenerator {
         ^

After each crash, upstart --user would restart unity8 (that's presumably the moment where the Ubuntu circle would flicker).

Commenting out the InputEventGenerator paragraph in /usr/share/unity8/Components/PhysicalKeysMapper.qml fixed the issue:

    //InputEventGenerator {
    // id: inputEventGenerator
    //}

    function onKeyPressed(event, currentEventTimestamp) {

root@alains-pocket-penguin:~# lsb_release -rd
Description: Ubuntu 15.04
Release: 15.04
root@alains-pocket-penguin:~# apt-cache policy unity8-common
unity8-common:
  Installed: 8.11+15.04.20160212-0ubuntu1
  Candidate: 8.11+15.04.20160212-0ubuntu1
  Version table:
 *** 8.11+15.04.20160212-0ubuntu1 0
       1001 http://ppa.launchpad.net/ci-train-ppa-service/emergency-snapshot/ubuntu/ vivid/main armhf Packages
        100 /var/lib/dpkg/status
     8.02+15.04.20150409.1-0ubuntu1 0
        500 http://ports.ubuntu.com/ubuntu-ports/ vivid/universe armhf Packages

What I expected to happen: after software upgrade, phone should boot up normally, giving me access to the UI

What happened instead: phone booted forever (Ubuntu circle showing), because it got in a loop with unity8 crashing and restarting forever...