Latest setup, test on oldest supported SDK (dart-lang/web_socket_channel#161)

pkgs/web_socket_channel/.github/workflows/test-package.yml

diff --git a/pkgs/web_socket_channel/.github/workflows/test-package.yml b/pkgs/web_socket_channel/.github/workflows/test-package.yml
index 3ddce8c..bba03d3 100644
--- a/pkgs/web_socket_channel/.github/workflows/test-package.yml
+++ b/pkgs/web_socket_channel/.github/workflows/test-package.yml
@@ -22,7 +22,7 @@
         sdk: [dev]
     steps:
       - uses: actions/checkout@v2
-      - uses: dart-lang/setup-dart@v0.5
+      - uses: dart-lang/setup-dart@v1.0
         with:
           sdk: ${{ matrix.sdk }}
       - id: install
@@ -42,11 +42,10 @@
       fail-fast: false
       matrix:
         os: [ubuntu-latest]
-        # TODO: beta -> 2.12.0 once it's stable!
-        sdk: [beta, dev]
+        sdk: [2.12.0, dev]
     steps:
       - uses: actions/checkout@v2
-      - uses: dart-lang/setup-dart@v0.5
+      - uses: dart-lang/setup-dart@v1.0
         with:
           sdk: ${{ matrix.sdk }}
       - id: install