Fix errant return (dart-lang/markdown#258)

pkgs/markdown/tool/stats.dart

diff --git a/pkgs/markdown/tool/stats.dart b/pkgs/markdown/tool/stats.dart
index 2cec692..3581579 100644
--- a/pkgs/markdown/tool/stats.dart
+++ b/pkgs/markdown/tool/stats.dart
@@ -105,7 +105,7 @@
 
   for (var entry in sections.entries) {
     if (specifiedSection != null && entry.key != specifiedSection) {
-      return;
+      continue;
     }
 
     final units = <DataCase>[];