Revert memoizer integration (#1594)

* Revert "Use the Memoizer class in place of ad-hoc caching in model.dart (#1577)"

This reverts commit be310b8.

* Merge error broke canonicalModelElement, fixed

* dartfmt

* Pin travis to 16 too

Author: jcollins-g

.travis.yml

@@ -1,7 +1,7 @@
 language: dart
 sudo: false
 dart:
-  - dev
+  - "dev/release/2.0.0-dev.16.0"
 env:
   - DARTDOC_BOT=main
   # TODO(devoncarew): add angulardart support