Avoid recreating BlockTag pattern on every access

pkgs/markdown/lib/src/block_parser.dart

diff --git a/pkgs/markdown/lib/src/block_parser.dart b/pkgs/markdown/lib/src/block_parser.dart
index 482ca35..65d78ba 100644
--- a/pkgs/markdown/lib/src/block_parser.dart
+++ b/pkgs/markdown/lib/src/block_parser.dart
@@ -485,7 +485,7 @@
 }
 
 class BlockTagBlockHtmlSyntax extends BlockHtmlSyntax {
-  RegExp get pattern => new RegExp(
+  static final _pattern = new RegExp(
       r'^ {0,3}</?(?:address|article|aside|base|basefont|blockquote|body|'
       r'caption|center|col|colgroup|dd|details|dialog|dir|div|dl|dt|fieldset|'
       r'figcaption|figure|footer|form|frame|frameset|h1|head|header|hr|html|'
@@ -494,6 +494,8 @@
       'title|tr|track|ul)'
       r'(?:\s|>|/>|$)');
 
+  RegExp get pattern => _pattern;
+
   const BlockTagBlockHtmlSyntax();
 
   Node parse(BlockParser parser) {