test against Dart 2.15

pkgs/pool/.github/workflows/ci.yml

diff --git a/pkgs/pool/.github/workflows/ci.yml b/pkgs/pool/.github/workflows/ci.yml
index 9a455e7..76d5352 100644
--- a/pkgs/pool/.github/workflows/ci.yml
+++ b/pkgs/pool/.github/workflows/ci.yml
@@ -47,7 +47,7 @@
       matrix:
         # Add macos-latest and/or windows-latest if relevant for this package.
         os: [ubuntu-latest]
-        sdk: [2.12.0, dev]
+        sdk: [2.15.0, dev]
     steps:
       - uses: actions/checkout@v2
       - uses: dart-lang/setup-dart@v1.0