Add a note to generated plugins files (#45557)

There has been some confusion about whether or not
.flutter-plugins-dependencies should be tracked in version control or
not. Added a comment to both it and .flutter-plugins explaining that
it's generated and shouldn't be.

.flutter-plugins-dependencies is parsed through JSON, and the JSON spec
doesn't support comments. So unfortunately the note is living in an
arbitrary "_info" key instead of an obvious top level comment.
diff --git a/packages/flutter_tools/test/general.shard/plugins_test.dart b/packages/flutter_tools/test/general.shard/plugins_test.dart
index 5d35408..0a84b8c 100644
--- a/packages/flutter_tools/test/general.shard/plugins_test.dart
+++ b/packages/flutter_tools/test/general.shard/plugins_test.dart
@@ -273,6 +273,7 @@
         expect(flutterProject.flutterPluginsFile.existsSync(), true);
         expect(flutterProject.flutterPluginsDependenciesFile.existsSync(), true);
         expect(flutterProject.flutterPluginsFile.readAsStringSync(),
+          '# This is a generated file; do not edit or check into version control.\n'
           'plugin-a=/.tmp_rand0/plugin.rand0/\n'
           'plugin-b=/.tmp_rand0/plugin.rand1/\n'
           'plugin-c=/.tmp_rand0/plugin.rand2/\n'
@@ -280,6 +281,7 @@
         );
         expect(flutterProject.flutterPluginsDependenciesFile.readAsStringSync(),
           '{'
+            '"_info":"// This is a generated file; do not edit or check into version control.",'
             '"dependencyGraph":['
               '{'
                 '"name":"plugin-a",'