adjust the min sdk we test against (dart-lang/source_maps#68)

diff --git a/pkgs/source_maps/.github/workflows/test-package.yml b/pkgs/source_maps/.github/workflows/test-package.yml
index 81ebfeb..d257b9f 100644
--- a/pkgs/source_maps/.github/workflows/test-package.yml
+++ b/pkgs/source_maps/.github/workflows/test-package.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: [2.18.0, dev]
     steps:
       - uses: actions/checkout@93ea575cb5d8a053eaa0ac8fa3b40d7e05a33cc8
       - uses: dart-lang/setup-dart@6a218f2413a3e78e9087f638a238f6b40893203d