Be gentler, for MacOS (#7684)

Turns out MacOS has a really low open files limit and so if you try to
open EVERY FILE AT THE SAME TIME it falls over.

This just opens the files one at a time, the way we used to back in
the old days.

dev/devicelab/bin/tasks/technical_debt__cost.dart

@@ -18,55 +18,21 @@ const double pythonCost = 3001.0; //  six average SWE days, in dollars
 final RegExp todoPattern = new RegExp(r'(?://|#) *TODO');
 final RegExp ignorePattern = new RegExp(r'// *ignore:');
 
-Stream<double> findCostsForFile(File file) {
+Future<double> findCostsForFile(File file) async {
   if (path.extension(file.path) == '.py')
-    return new Stream<double>.fromIterable(<double>[pythonCost]);
+    return pythonCost;
   if (path.extension(file.path) != '.dart' &&
       path.extension(file.path) != '.yaml' &&
       path.extension(file.path) != '.sh')
-    return null;
-  StreamController<double> result = new StreamController<double>();
-  file.openRead().transform(UTF8.decoder).transform(const LineSplitter()).listen((String line) {
+    return 0.0;
+  double total = 0.0;
+  for (String line in await file.readAsLines()) {
     if (line.contains(todoPattern))
-      result.add(todoCost);
+      total += todoCost;
     if (line.contains(ignorePattern))
-      result.add(ignoreCost);
-  }, onDone: () { result.close(); });
-  return result.stream;
-}
-
-Stream<double> findCostsForDirectory(Directory directory, Set<String> gitFiles) {
-  StreamController<double> result = new StreamController<double>();
-  Set<StreamSubscription<dynamic>> subscriptions = new Set<StreamSubscription<dynamic>>();
-
-  void checkDone(StreamSubscription<dynamic> subscription, String path) {
-    subscriptions.remove(subscription);
-    if (subscriptions.isEmpty)
-      result.close();
+      total += ignoreCost;
   }
-
-  StreamSubscription<FileSystemEntity> listSubscription;
-  subscriptions.add(listSubscription = directory.list(followLinks: false).listen((FileSystemEntity entity) {
-    String name = path.relative(entity.path, from: flutterDirectory.path);
-    if (gitFiles.contains(name)) {
-      if (entity is File) {
-        StreamSubscription<double> subscription;
-        subscription = findCostsForFile(entity)?.listen((double cost) {
-          result.add(cost);
-        }, onDone: () { checkDone(subscription, name); });
-        if (subscription != null)
-          subscriptions.add(subscription);
-      } else if (entity is Directory) {
-        StreamSubscription<double> subscription;
-        subscription = findCostsForDirectory(entity, gitFiles)?.listen((double cost) {
-          result.add(cost);
-        }, onDone: () { checkDone(subscription, name); });
-        if (subscription != null)
-          subscriptions.add(subscription);
-      }
-    }
-  }, onDone: () { checkDone(listSubscription, directory.path); }));
-  return result.stream;
+  return total;
 }
 
 const String _kBenchmarkKey = 'technical_debt_in_dollars';
@@ -78,21 +44,12 @@ Future<Null> main() async {
       <String>['ls-files', '--full-name', flutterDirectory.path],
       workingDirectory: flutterDirectory.path,
     );
-    Set<String> gitFiles = new Set<String>();
-    await for (String entry in git.stdout.transform(UTF8.decoder).transform(const LineSplitter())) {
-      String subentry = '';
-      for (String component in path.split(entry)) {
-        if (subentry.isNotEmpty)
-          subentry += path.separator;
-        subentry += component;
-        gitFiles.add(subentry);
-      }
-    }
+    double total = 0.0;
+    await for (String entry in git.stdout.transform(UTF8.decoder).transform(const LineSplitter()))
+      total += await findCostsForFile(new File(path.join(flutterDirectory.path, entry)));
     int gitExitCode = await git.exitCode;
     if (gitExitCode != 0)
       throw new Exception('git exit with unexpected error code $gitExitCode');
-    List<double> costs = await findCostsForDirectory(flutterDirectory, gitFiles).toList();
-    double total = costs.fold(0.0, (double total, double cost) => total + cost);
     return new TaskResult.success(
       <String, dynamic>{_kBenchmarkKey: total},
       benchmarkScoreKeys: <String>[_kBenchmarkKey],