relax SDK lower-bound (#2545)

* relax SDK lower-bound

* fmt
diff --git a/pubspec.yaml b/pubspec.yaml
index b36da1d..deb0bb8 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -9,7 +9,7 @@
 documentation: https://dart-lang.github.io/linter/lints
 
 environment:
-  sdk: '>=2.13.0-0 <3.0.0'
+  sdk: '>=2.12.0-0 <3.0.0'
 
 dependencies:
   analyzer: ^1.1.0
diff --git a/test/_data/public_member_api_docs/pubspec.yaml b/test/_data/public_member_api_docs/pubspec.yaml
index 6cb8696..42eac93 100644
--- a/test/_data/public_member_api_docs/pubspec.yaml
+++ b/test/_data/public_member_api_docs/pubspec.yaml
@@ -1 +1,4 @@
 name: public_member_api_docs
+
+environment:
+  sdk: '>=2.13.0-0 <3.0.0'
diff --git a/test/integration/public_member_api_docs.dart b/test/integration/public_member_api_docs.dart
index b6ad794..3c66332 100644
--- a/test/integration/public_member_api_docs.dart
+++ b/test/integration/public_member_api_docs.dart
@@ -9,6 +9,15 @@
 void main() {
   group('public_member_api_docs', () {
     test('lint lib/ sources and non-lib/ sources', () async {
+      var pubResult = Process.runSync(
+          'dart',
+          [
+            'pub',
+            'get',
+          ],
+          workingDirectory: 'test/_data/public_member_api_docs');
+      expect(pubResult.exitCode, 0);
+
       var result = Process.runSync(
         'dart',
         ['analyze', 'test/_data/public_member_api_docs'],