Add union types to the analysis server API spec.

pkg/analysis_server/tool/spec/to_html.dart

Index: pkg/analysis_server/tool/spec/to_html.dart
diff --git a/pkg/analysis_server/tool/spec/to_html.dart b/pkg/analysis_server/tool/spec/to_html.dart
index a1d53222c64e0b6a82cf120ad11b486e92eb71ee..ef18f00f15d0d8e5612a345e4321611a541f5186 100644
--- a/pkg/analysis_server/tool/spec/to_html.dart
+++ b/pkg/analysis_server/tool/spec/to_html.dart
@@ -195,6 +195,18 @@ class TypeVisitor extends HierarchicalApiVisitor with HtmlMixin, HtmlCodeGenerat
       write(displayName);
     }
   }
+
+  @override
+  void visitTypeUnion(TypeUnion typeUnion) {
+    bool verticalBarNeeded = false;
+    for (TypeDecl choice in typeUnion.choices) {
+      if (verticalBarNeeded) {
+        write(' | ');
+      }
+      visitTypeDecl(choice);
+      verticalBarNeeded = true;
+    }
+  }
 }
 
 /**