Migrate from no-implicit-casts to strict-casts (dart-lang/timing#24)

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

* Bump GH actions from Dart 2.14 to 2.18.
diff --git a/pkgs/timing/.github/workflows/test-package.yml b/pkgs/timing/.github/workflows/test-package.yml
index 63a6491..5b99fc7 100644
--- a/pkgs/timing/.github/workflows/test-package.yml
+++ b/pkgs/timing/.github/workflows/test-package.yml
@@ -20,7 +20,7 @@
     strategy:
       fail-fast: false
       matrix:
-        sdk: [2.14.0, dev]
+        sdk: [2.18.0, dev]
     steps:
       - uses: actions/checkout@755da8c3cf115ac066823e79a1e1788f8940201b
       - uses: dart-lang/setup-dart@6a218f2413a3e78e9087f638a238f6b40893203d
@@ -44,7 +44,7 @@
       matrix:
         # Add macos-latest and/or windows-latest if relevant for this package.
         os: [ubuntu-latest]
-        sdk: [2.14.0, dev]
+        sdk: [2.18.0, dev]
     steps:
       - uses: actions/checkout@755da8c3cf115ac066823e79a1e1788f8940201b
       - uses: dart-lang/setup-dart@6a218f2413a3e78e9087f638a238f6b40893203d
diff --git a/pkgs/timing/analysis_options.yaml b/pkgs/timing/analysis_options.yaml
index dd1616f..0a90722 100644
--- a/pkgs/timing/analysis_options.yaml
+++ b/pkgs/timing/analysis_options.yaml
@@ -1,8 +1,8 @@
 include: package:lints/recommended.yaml
 
 analyzer:
-  strong-mode:
-    implicit-casts: false
+  language:
+    strict-casts: true
 
 linter:
   rules: