Use a smaller set of github extensions for markdown (#2002)

* Use a smaller set of github extensions for markdown

* Explicit add of the github extensions for markdown

Author: keertip

lib/src/markdown_processor.dart

@@ -141,11 +141,19 @@ final HtmlEscape htmlEscape = const HtmlEscape(HtmlEscapeMode.element);
 
 final List<md.InlineSyntax> _markdown_syntaxes = [
   new _InlineCodeSyntax(),
-  new _AutolinkWithoutScheme()
-]..addAll(md.ExtensionSet.gitHubWeb.inlineSyntaxes);
+  new _AutolinkWithoutScheme(),
+  md.InlineHtmlSyntax(),
+  md.StrikethroughSyntax(),
+  md.AutolinkExtensionSyntax(),
+];
+
+final List<md.BlockSyntax> _markdown_block_syntaxes = [
+  const md.FencedCodeBlockSyntax(),
+  const md.HeaderWithIdSyntax(),
+  const md.SetextHeaderWithIdSyntax(),
+  const md.TableSyntax(),
 
-final List<md.BlockSyntax> _markdown_block_syntaxes = []
-  ..addAll(md.ExtensionSet.gitHubWeb.blockSyntaxes);
+];
 
 // Remove these schemas from the display text for hyperlinks.
 final RegExp _hide_schemes = new RegExp('^(http|https)://');

test/model_test.dart

@@ -1340,6 +1340,16 @@ void main() {
                 '<tbody><tr><td><a href="fake/DocumentWithATable/foo-constant.html">foo</a></td>'),
             isTrue);
       });
+
+      test('Verify there is no emoji support', () {
+        TopLevelVariable tpvar = fakeLibrary.constants.firstWhere((t) => t.name == 'hasMarkdownInDoc');
+        docsAsHtml = tpvar.documentationAsHtml;
+        expect(
+            docsAsHtml.contains(
+                '3ffe:2a00:100:7031::1'),
+            isTrue);
+      });
+
     });
 
     group('doc references', () {

testing/test_package/lib/fake.dart

@@ -70,6 +70,9 @@ export 'src/tool.dart';
 // ignore: uri_does_not_exist
 export 'package:test_package/fake.dart';
 
+/// Does not render with emoji 3ffe:2a00:100:7031::1
+const int hasMarkdownInDoc = 1;
+
 abstract class ImplementingThingy implements BaseThingy {}
 
 abstract class BaseThingy {