Update CI (#47)

diff --git a/.github/workflows/test-package.yml b/.github/workflows/test-package.yml
index 0a2a874..2f5a52c 100644
--- a/.github/workflows/test-package.yml
+++ b/.github/workflows/test-package.yml
@@ -23,7 +23,7 @@
         sdk: [dev]
     steps:
       - uses: actions/checkout@v2
-      - uses: dart-lang/setup-dart@v0.3
+      - uses: dart-lang/setup-dart@v1.0
         with:
           sdk: ${{ matrix.sdk }}
       - id: install
@@ -36,21 +36,17 @@
         run: dart analyze --fatal-infos
         if: always() && steps.install.outcome == 'success'
 
-  # Run tests on a matrix consisting of two dimensions:
-  # 1. OS: ubuntu-latest, (macos-latest, windows-latest)
-  # 2. release channel: dev
   test:
     needs: analyze
     runs-on: ${{ matrix.os }}
     strategy:
       fail-fast: false
       matrix:
-        # Add macos-latest and/or windows-latest if relevant for this package.
         os: [ubuntu-latest]
-        sdk: [dev]
+        sdk: [2.12.0, dev]
     steps:
       - uses: actions/checkout@v2
-      - uses: dart-lang/setup-dart@v0.3
+      - uses: dart-lang/setup-dart@v1.0
         with:
           sdk: ${{ matrix.sdk }}
       - id: install