Add union types to the analysis server API spec.

pkg/analysis_server/tool/spec/codegen_matchers.dart

Index: pkg/analysis_server/tool/spec/codegen_matchers.dart
diff --git a/pkg/analysis_server/tool/spec/codegen_matchers.dart b/pkg/analysis_server/tool/spec/codegen_matchers.dart
index e2e62fb949282959a9896098bef886a5e9e00185..d6de74b86104a8a9205e548fbd6d72e2427d1228 100644
--- a/pkg/analysis_server/tool/spec/codegen_matchers.dart
+++ b/pkg/analysis_server/tool/spec/codegen_matchers.dart
@@ -187,6 +187,20 @@ class CodegenMatchersVisitor extends HierarchicalApiVisitor with CodeGenerator {
   void visitTypeReference(TypeReference typeReference) {
     write(camelJoin(['is', typeReference.typeName]));
   }
+
+  @override
+  void visitTypeUnion(TypeUnion typeUnion) {
+    bool commaNeeded = false;
+    write('isOneOf([');
+    for (TypeDecl choice in typeUnion.choices) {
+      if (commaNeeded) {
+        write(', ');
+      }
+      visitTypeDecl(choice);
+      commaNeeded = true;
+    }
+    write('])');
+  }
 }
 
 /**