Migrate from no-implicit-casts to strict-casts (dart-archive/io#94)

* Migrate from no-implicit-dynamic to strict-casts

* Bump CI to Dart 2.18.0

Author: srawlins

pkgs/io/.github/workflows/ci.yml

@@ -20,7 +20,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        sdk: [dev, 2.12.0]
+        sdk: [dev, 2.18.0]
     steps:
       - uses: actions/checkout@755da8c3cf115ac066823e79a1e1788f8940201b
       - uses: dart-lang/setup-dart@6a218f2413a3e78e9087f638a238f6b40893203d
@@ -47,7 +47,7 @@ jobs:
       matrix:
         # Add macos-latest and/or windows-latest if relevant for this package.
         os: [ubuntu-latest]
-        sdk: [dev, 2.12.0]
+        sdk: [dev, 2.18.0]
     steps:
       - uses: actions/checkout@755da8c3cf115ac066823e79a1e1788f8940201b
       - uses: dart-lang/setup-dart@6a218f2413a3e78e9087f638a238f6b40893203d

pkgs/io/analysis_options.yaml

@@ -1,10 +1,8 @@
 include: package:lints/recommended.yaml
 
 analyzer:
-  strong-mode:
-    implicit-casts: false
-    implicit-dynamic: false
   language:
+    strict-casts: true
     strict-raw-types: true
 
 linter: