Migrate to null safety (#139)

- Append `-nullsafety.0` to version.
- Set min SDK to `2.12.0-0`.
- Use late field initialization over constructor assignment to clean up
  the handling of backreferences in the different "phases".
- Remove some conditional branches that are impossible to hit.
- Make the non-nullable override of Text nodes explicit with `covariant`.

.github/workflows/test-package.yml

@@ -2,9 +2,9 @@ name: Dart CI
 
 on:
   push:
-    branches: [ master ]
+    branches: [master]
   pull_request:
-    branches: [ master ]
+    branches: [master]
   schedule:
     - cron: "0 0 * * 0"
 
@@ -35,7 +35,7 @@ jobs:
       fail-fast: false
       matrix:
         os: [ubuntu-latest]
-        sdk: [2.8.1, dev]
+        sdk: [dev]
     steps:
       - uses: actions/checkout@v2
       - uses: dart-lang/setup-dart@v0.3

CHANGELOG.md

@@ -1,5 +1,6 @@
-## 0.15.0-dev
+## 0.15.0-nullsafety.0-dev
 
+- Migrate to null safety.
 - Drop `lastPhase`, `beforeRcDataPhase`, and `container` fields from
   `HtmlParser` class. These fields never had a value other than `null`.