Cleanup test-package.yml (#47)

diff --git a/.github/workflows/test-package.yml b/.github/workflows/test-package.yml
index e55702c..02921f3 100644
--- a/.github/workflows/test-package.yml
+++ b/.github/workflows/test-package.yml
@@ -14,18 +14,18 @@
 
 jobs:
   # Check code formatting and static analysis on a single OS (linux)
-  # against Dart dev.
+  # against dev, stable, and 2.12.0 (the package's lower bound).
   analyze:
     runs-on: ubuntu-latest
     strategy:
       fail-fast: false
       matrix:
-        sdk: [dev]
+        sdk: [dev, stable, 2.12.0]
     steps:
       - uses: actions/checkout@v2
-      - uses: dart-lang/setup-dart@v0.1
+      - uses: dart-lang/setup-dart@v1.0
         with:
-          channel: ${{ matrix.sdk }}
+          sdk: ${{ matrix.sdk }}
       - id: install
         name: Install dependencies
         run: dart pub get
@@ -37,22 +37,21 @@
         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
+  # 1. OS: ubuntu-latest
+  # 2. Release channel: dev, stable, and 2.12.0 (the package's lower bound)
   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: [dev, stable, 2.12.0]
     steps:
       - uses: actions/checkout@v2
-      - uses: dart-lang/setup-dart@v0.1
+      - uses: dart-lang/setup-dart@v1.0
         with:
-          channel: ${{ matrix.sdk }}
+          sdk: ${{ matrix.sdk }}
       - id: install
         name: Install dependencies
         run: dart pub get