A variety of cleanups to bring our samples up-to-date.

samples/markdown/block_parser.dart

Index: samples/markdown/block_parser.dart
===================================================================
--- samples/markdown/block_parser.dart	(revision 12654)
+++ samples/markdown/block_parser.dart	(working copy)
@@ -20,8 +20,8 @@
 /// Three or more hyphens, asterisks or underscores by themselves. Note that
 /// a line like `----` is valid as both HR and SETEXT. In case of a tie,
 /// SETEXT should win.
-const _RE_HR = const RegExp(@'^[ ]{0,3}((-+[ ]{0,2}){3,}|' +
-                                 @'(_+[ ]{0,2}){3,}|' +
+const _RE_HR = const RegExp(@'^[ ]{0,3}((-+[ ]{0,2}){3,}|'
+                                 @'(_+[ ]{0,2}){3,}|'
                                  @'(\*+[ ]{0,2}){3,})$');
 
 /// Really hacky way to detect block-level embedded HTML. Just looks for