Generate docgen file names with hyphens in their names instead of colons

pkg/docgen/lib/src/generator.dart

Index: pkg/docgen/lib/src/generator.dart
diff --git a/pkg/docgen/lib/src/generator.dart b/pkg/docgen/lib/src/generator.dart
index 2b7b1f876275ef90352503001f166dddde98a399..3a050419cedb713496060f5344b1eb93adbeec37 100644
--- a/pkg/docgen/lib/src/generator.dart
+++ b/pkg/docgen/lib/src/generator.dart
@@ -247,7 +247,7 @@ void _writeOutputFiles(Map<String, dynamic> libraryMap, Iterable<Indexable>
 
 /// Helper method to serialize the given Indexable out to a file.
 void _writeIndexableToFile(Indexable result, JsonEncoder encoder) {
-  var outputFile = result.qualifiedName + '.json';
+  var outputFile = result.fileName + '.json';
   var output = encoder.convert(result.toMap());
   _writeToFile(output, outputFile);
 }