Add output-size info to the '--stats' flag on various tasks. (#3879)

The '--stats' flag currently wraps commands with a call to `time`, and passes a
flag to `dartdoc.dart`. This adds a call to `du` in order to report how many MB
are generated by dartdoc.

Author: srawlins

tool/task.dart

@@ -566,7 +566,7 @@ Future<Iterable<Map<String, Object?>>> _docSdk({
   var launcher = SubprocessLauncher('build-sdk-docs');
   await launcher
       .runStreamedDartCommand(['pub', 'get'], workingDirectory: dartdocPath);
-  return await launcher.runStreamedDartCommand(
+  var output = await launcher.runStreamedDartCommand(
     [
       '--enable-asserts',
       path.join('bin', 'dartdoc.dart'),
@@ -587,6 +587,19 @@ Future<Iterable<Map<String, Object?>>> _docSdk({
     workingDirectory: dartdocPath,
     withStats: withStats,
   );
+  if (withStats && (Platform.isLinux || Platform.isMacOS)) {
+    var diskUsageResult = Process.runSync('du', ['-sh', sdkDocsPath]);
+    // Output looks like
+    // 146M    /var/folders/72/ltck4q353hsg3bn8kpkg7f84005w15/T/sdkdocsHcquiB
+    var diskUsageNumber =
+        (diskUsageResult.stdout as String).trim().split(RegExp('\\s+')).first;
+
+    // Prints all files in `sdkDocsPath`, newline-separated.
+    var findResult = Process.runSync('find', [sdkDocsPath, '-type', 'f']);
+    var fileCount = (findResult.stdout as String).split('\n').length;
+    print('Generated $fileCount files amounting to $diskUsageNumber.');
+  }
+  return output;
 }
 
 /// Creates a clean version of dartdoc (based on the current directory, assumed