[BUGFIX] ember intends on making length non-settable

[runspired]

cc @rwjblue @krisselden

see emberjs/ember.js#16776 for reference

I believe we should backport this all the way to 2.18 LTS

[rwjblue]

These failures seem legit:

354) [Chrome 67.0] unit/record_array - DS.RecordArray: stops updating when destroyed
     Has no more records
     expected: 0
       actual: 1
         at http://localhost:7357/assets/tests.js:9970:14
         at Backburner._run (http://localhost:7357/assets/vendor.js:21220:35)
         at Backburner.run (http://localhost:7357/assets/vendor.js:20930:25)
         at Object.run (http://localhost:7357/assets/vendor.js:37594:29)
         at Object.<anonymous> (http://localhost:7357/assets/tests.js:9969:11)
         at runTest (http://localhost:7357/assets/test-support.js:4199:30)

1404) [Chrome 67.0] unit/record-arrays/record-array - DS.RecordArray: #destroy
     expected: null
       actual: [ [length]: 0 ]
         at Object.<anonymous> (http://localhost:7357/assets/tests.js:45809:12)
         at runTest (http://localhost:7357/assets/test-support.js:4199:30)
         at Test.run (http://localhost:7357/assets/test-support.js:4185:6)
         at http://localhost:7357/assets/test-support.js:4406:12
         at advanceTaskQueue (http://localhost:7357/assets/test-support.js:3798:6)
         at advance (http://localhost:7357/assets/test-support.js:3779:4)
1406) [Chrome 67.0] unit/record-arrays/record-array - DS.RecordArray: #destroy 
     expected: null
       actual: [ [length]: 0 ]
         at Object.<anonymous> (http://localhost:7357/assets/tests.js:45874:12)
         at runTest (http://localhost:7357/assets/test-support.js:4199:30)
         at Test.run (http://localhost:7357/assets/test-support.js:4185:6)
         at http://localhost:7357/assets/test-support.js:4406:12
         at advanceTaskQueue (http://localhost:7357/assets/test-support.js:3798:6)
         at advance (http://localhost:7357/assets/test-support.js:3779:4)

[rwjblue]

emberjs/ember.js#16783 should make this PR not required, but we should likely still land it 😸

[rwjblue]

Fixed on the Ember side.