Code cleanup and scaffolding for new Extendable mixin (dart-lang#2044)

* Add some tests to trigger crash bug

* Add warning for not yet implemented and seal off comment resolution to avoid type errors

* Begin implementation

* Extend a timeout

* warnable

* Main tests pass

* Finish up scaffolding and clean up

* Remove an import that got added somewhere

Author: jcollins-g

bin/dartdoc.dart

@@ -37,7 +37,7 @@ Future<List<DartdocOption>> createDartdocProgramOptions() async {
     DartdocOptionArgOnly<bool>('help', false,
         abbr: 'h', help: 'Show command help.', negatable: false),
     DartdocOptionArgOnly<bool>('version', false,
-        help: 'Display the version for $name.', negatable: false),
+        help: 'Display the version for $programName.', negatable: false),
   ];
 }
 

lib/dartdoc.dart

@@ -32,7 +32,7 @@ export 'package:dartdoc/src/generator.dart';
 export 'package:dartdoc/src/model.dart';
 export 'package:dartdoc/src/package_meta.dart';
 
-const String name = 'dartdoc';
+const String programName = 'dartdoc';
 // Update when pubspec version changes by running `pub run build_runner build`
 const String dartdocVersion = packageVersion;
 

lib/src/markdown_processor.dart

@@ -650,8 +650,7 @@ class _MarkdownCommentReference {
           (element as Inheritable).overriddenElement;
       while (overriddenElement != null) {
         tryClasses.add(
-            ((element as Inheritable).overriddenElement as EnclosedElement)
-                .enclosingElement);
+            (element as Inheritable).overriddenElement.enclosingElement);
         overriddenElement = overriddenElement.overriddenElement;
       }
     }