Fix test failure by silencing analyzer preamble and congrats (#14745)

dev/devicelab/bin/tasks/dartdocs.dart

@@ -15,7 +15,7 @@ Future<Null> main() async {
     final Stopwatch clock = new Stopwatch()..start();
     final Process analysis = await startProcess(
       path.join(flutterDirectory.path, 'bin', 'flutter'),
-      <String>['analyze', '--flutter-repo', '--dartdocs'],
+      <String>['analyze', '--no-preamble', '--no-congratulate', '--flutter-repo', '--dartdocs'],
       workingDirectory: flutterDirectory.path,
     );
     int publicMembers = 0;
@@ -37,10 +37,6 @@ Future<Null> main() async {
       print('analyzer stdout: $entry');
       if (entry == 'Building flutter tool...') {
         // ignore this line
-      } else if (entry.startsWith('Analyzing ')) {
-        // ignore this line
-      } else if (entry.startsWith('No analyzer warnings!')) {
-        // ignore this line
       } else {
         otherLines += 1;
       }