Code generate Dart classes representing the analysis server API.

pkg/analysis_server/tool/spec/from_html.dart

Index: pkg/analysis_server/tool/spec/from_html.dart
diff --git a/pkg/analysis_server/tool/spec/from_html.dart b/pkg/analysis_server/tool/spec/from_html.dart
index c1513b8e14908fbdb0f985db266c2ce7e9532649..d3b201aa4b388c1476f23794ed36865764010bcd 100644
--- a/pkg/analysis_server/tool/spec/from_html.dart
+++ b/pkg/analysis_server/tool/spec/from_html.dart
@@ -388,8 +388,9 @@ List<TypeDecl> processContentsAsTypes(dom.Element html) {
       types.add(new TypeReference(innerText(child), child));
     },
     'union': (dom.Element child) {
-      checkAttributes(child, []);
-      types.add(new TypeUnion(processContentsAsTypes(child), child));
+      checkAttributes(child, ['field']);
+      String field = child.attributes['field'];
+      types.add(new TypeUnion(processContentsAsTypes(child), field, child));
     }
   });
   return types;