Make Syntax const

pkgs/markdown/lib/src/markdown/block_parser.dart

diff --git a/pkgs/markdown/lib/src/markdown/block_parser.dart b/pkgs/markdown/lib/src/markdown/block_parser.dart
index d163e9c..a12033d 100644
--- a/pkgs/markdown/lib/src/markdown/block_parser.dart
+++ b/pkgs/markdown/lib/src/markdown/block_parser.dart
@@ -88,28 +88,21 @@
 abstract class BlockSyntax {
   /// Gets the collection of built-in block parsers. To turn a series of lines
   /// into blocks, each of these will be tried in turn. Order matters here.
-  static List<BlockSyntax> get syntaxes {
-    // Lazy initialize.
-    if (_syntaxes == null) {
-      _syntaxes = [
-          new EmptyBlockSyntax(),
-          new BlockHtmlSyntax(),
-          new SetextHeaderSyntax(),
-          new HeaderSyntax(),
-          new CodeBlockSyntax(),
-          new FencedCodeBlockSyntax(),
-          new BlockquoteSyntax(),
-          new HorizontalRuleSyntax(),
-          new UnorderedListSyntax(),
-          new OrderedListSyntax(),
-          new ParagraphSyntax()
-        ];
-    }
+  static const List<BlockSyntax> syntaxes = const[
+    const EmptyBlockSyntax(),
+    const BlockHtmlSyntax(),
+    const SetextHeaderSyntax(),
+    const HeaderSyntax(),
+    const CodeBlockSyntax(),
+    const FencedCodeBlockSyntax(),
+    const BlockquoteSyntax(),
+    const HorizontalRuleSyntax(),
+    const UnorderedListSyntax(),
+    const OrderedListSyntax(),
+    const ParagraphSyntax()
+  ];
 
-    return _syntaxes;
-  }
-
-  static List<BlockSyntax> _syntaxes;
+  const BlockSyntax();
 
   /// Gets the regex used to identify the beginning of this block, if any.
   RegExp get pattern => null;
@@ -146,6 +139,8 @@
 class EmptyBlockSyntax extends BlockSyntax {
   RegExp get pattern => _RE_EMPTY;
 
+  const EmptyBlockSyntax();
+
   Node parse(BlockParser parser) {
     parser.advance();
 
@@ -156,6 +151,9 @@
 
 /// Parses setext-style headers.
 class SetextHeaderSyntax extends BlockSyntax {
+
+  const SetextHeaderSyntax();
+
   bool canParse(BlockParser parser) {
     // Note: matches *next* line, not the current one. We're looking for the
     // underlining after this line.
@@ -178,6 +176,8 @@
 class HeaderSyntax extends BlockSyntax {
   RegExp get pattern => _RE_HEADER;
 
+  const HeaderSyntax();
+
   Node parse(BlockParser parser) {
     final match = pattern.firstMatch(parser.current);
     parser.advance();
@@ -191,6 +191,8 @@
 class BlockquoteSyntax extends BlockSyntax {
   RegExp get pattern => _RE_BLOCKQUOTE;
 
+  const BlockquoteSyntax();
+
   Node parse(BlockParser parser) {
     final childLines = parseChildLines(parser);
 
@@ -205,6 +207,8 @@
 class CodeBlockSyntax extends BlockSyntax {
   RegExp get pattern => _RE_INDENT;
 
+  const CodeBlockSyntax();
+
   List<String> parseChildLines(BlockParser parser) {
     final childLines = <String>[];
 
@@ -249,7 +253,11 @@
 class FencedCodeBlockSyntax extends BlockSyntax {
   RegExp get pattern => _RE_CODE;
 
-  List<String> parseChildLines(BlockParser parser, String endBlock) {
+  const FencedCodeBlockSyntax();
+
+  List<String> parseChildLines(BlockParser parser, [String endBlock]) {
+    if(endBlock == null) endBlock = '';
+
     final childLines = <String>[];
     parser.advance();
     while (!parser.isDone) {
@@ -285,16 +293,14 @@
     }
     return element;
   }
-
-  void _addElementAttributes(Element element) {
-
-  }
 }
 
 /// Parses horizontal rules like `---`, `_ _ _`, `*  *  *`, etc.
 class HorizontalRuleSyntax extends BlockSyntax {
   RegExp get pattern => _RE_HR;
 
+  const HorizontalRuleSyntax();
+
   Node parse(BlockParser parser) {
     final match = pattern.firstMatch(parser.current);
     parser.advance();
@@ -317,6 +323,8 @@
 
   bool get canEndBlock => false;
 
+  const BlockHtmlSyntax();
+
   Node parse(BlockParser parser) {
     final childLines = [];
 
@@ -343,6 +351,8 @@
 
   String get listTag;
 
+  const ListSyntax();
+
   Node parse(BlockParser parser) {
     final items = <ListItem>[];
     var childLines = <String>[];
@@ -485,18 +495,24 @@
 class UnorderedListSyntax extends ListSyntax {
   RegExp get pattern => _RE_UL;
   String get listTag => 'ul';
+
+  const UnorderedListSyntax();
 }
 
 /// Parses ordered lists.
 class OrderedListSyntax extends ListSyntax {
   RegExp get pattern => _RE_OL;
   String get listTag => 'ol';
+
+  const OrderedListSyntax();
 }
 
 /// Parses paragraphs of regular text.
 class ParagraphSyntax extends BlockSyntax {
   bool get canEndBlock => false;
 
+  const ParagraphSyntax();
+
   bool canParse(BlockParser parser) => true;
 
   Node parse(BlockParser parser) {