Add union types to the analysis server API spec.

pkg/analysis_server/tool/spec/api.dart

Index: pkg/analysis_server/tool/spec/api.dart
diff --git a/pkg/analysis_server/tool/spec/api.dart b/pkg/analysis_server/tool/spec/api.dart
index 5b70f1449015a9cb5b55ef7ab961454bd333842d..7c01e32f2914748eebdcf0f23161603bafa98015 100644
--- a/pkg/analysis_server/tool/spec/api.dart
+++ b/pkg/analysis_server/tool/spec/api.dart
@@ -17,6 +17,7 @@ import 'package:html5lib/dom.dart' as dom;
  */
 abstract class ApiVisitor<T> {
   T visitTypeReference(TypeReference typeReference);
+  T visitTypeUnion(TypeUnion typeUnion);
   T visitTypeObject(TypeObject typeObject);
   T visitTypeList(TypeList typeList);
   T visitTypeMap(TypeMap typeMap);
@@ -118,6 +119,11 @@ class HierarchicalApiVisitor extends ApiVisitor {
   void visitTypeReference(TypeReference typeReference) {
   }
 
+  @override
+  void visitTypeUnion(TypeUnion typeUnion) {
+    typeUnion.choices.forEach(visitTypeDecl);
+  }
+
   /**
    * If [type] is a [TypeReference] that is defined in the API, follow the
    * chain until a non-[TypeReference] is found, if possible.
@@ -366,6 +372,17 @@ class TypeReference extends TypeDecl {
 }
 
 /**
+ * Type which represents a union among multiple choices.
+ */
+class TypeUnion extends TypeDecl {
+  final List<TypeDecl> choices;
+
+  TypeUnion(this.choices, dom.Element html) : super(html);
+
+  accept(ApiVisitor visitor) => visitor.visitTypeUnion(this);
+}
+
+/**
  * Type of a JSON object with specified fields, some of which may be optional.
  */
 class TypeObject extends TypeDecl {