Revert "drop legacy SDk tests" - Test on dart 2.8.4 (#15)

This reverts commit d6fdffe1c6c3e1bcbe80dcab92e0ab830eb75d48.
diff --git a/.github/workflows/test-package.yml b/.github/workflows/test-package.yml
index 21a3c50..bd3c848 100644
--- a/.github/workflows/test-package.yml
+++ b/.github/workflows/test-package.yml
@@ -59,3 +59,27 @@
       - name: Run VM tests
         run: dart test --platform vm
         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: 2.8.0
+  test-legacy-sdk:
+    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: [2.8.4]
+    steps:
+      - uses: actions/checkout@v2
+      - uses: dart-lang/setup-dart@v0.3
+        with:
+          sdk: ${{ matrix.sdk }}
+      - id: install
+        name: Install dependencies
+        run: pub get
+      - name: Run VM tests
+        run: pub run test --platform vm
+        if: always() && steps.install.outcome == 'success'