Bump SDK to 2.19

Analyzer 5.7.0 is using dart:io classes which were added in 2.19.

Author: mraleph

.github/workflows/test-package.yml

@@ -20,7 +20,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        sdk: [2.18.0, dev]
+        sdk: [2.19.0, dev]
     steps:
       - uses: actions/checkout@ac593985615ec2ede58e132d2e21d2b1cbd6127c
       - uses: dart-lang/setup-dart@a57a6c04cf7d4840e88432aad6281d1e125f0d46
@@ -50,7 +50,7 @@ jobs:
       matrix:
         # Add macos-latest and/or windows-latest if relevant for this package.
         os: [ubuntu-latest]
-        sdk: [2.18.0, dev]
+        sdk: [2.19.0, dev]
     steps:
       - uses: actions/checkout@ac593985615ec2ede58e132d2e21d2b1cbd6127c
       - uses: dart-lang/setup-dart@a57a6c04cf7d4840e88432aad6281d1e125f0d46