run pub get from 2.7 instead of latest

Author: lohanidamodar

tests/SDKTest.php

@@ -37,7 +37,7 @@ class SDKTest extends TestCase
             'build' => [
                 'mkdir -p tests/sdks/dart/tests',
                 'cp tests/languages/dart/tests.dart tests/sdks/dart/tests/tests.dart',
-                'docker run --rm -v $(pwd):/app -w /app/tests/sdks/dart --env PUB_CACHE=vendor google/dart:latest pub get',
+                'docker run --rm -v $(pwd):/app -w /app/tests/sdks/dart --env PUB_CACHE=vendor google/dart:2.7 pub get',
             ],
             'envs' => [
                 // 'dart-2.6' => 'docker run --rm -v $(pwd):/app -w /app/tests/sdks/dart --env PUB_CACHE=vendor google/dart:2.6 pub run tests/tests.dart',