added docker 2.12

Author: lohanidamodar

tests/SDKTest.php

@@ -53,6 +53,7 @@ class SDKTest extends TestCase
                 'cp tests/languages/dart/tests.dart tests/sdks/dart/tests/tests.dart',
             ],
             'envs' => [
+                'dart-2.12' => 'docker run --rm -v $(pwd):/app -w /app/tests/sdks/dart dart:2.12 sh -c "dart pub get && dart pub run tests/tests.dart"',
                 'dart-stable' => 'docker run --rm -v $(pwd):/app -w /app/tests/sdks/dart dart:stable sh -c "dart pub get && dart pub run tests/tests.dart"',
                 'dart-beta' => 'docker run --rm -v $(pwd):/app -w /app/tests/sdks/dart dart:beta sh -c "dart pub get && dart pub run tests/tests.dart"',
             ],