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

pkg/docgen/test/inherited_comments_test.dart

Index: pkg/docgen/test/inherited_comments_test.dart
diff --git a/pkg/docgen/test/inherited_comments_test.dart b/pkg/docgen/test/inherited_comments_test.dart
index 800bac6ba5520710171c24acad084c3644d3366d..3f078871168b580379840de7f80e17b6b50ab133 100644
--- a/pkg/docgen/test/inherited_comments_test.dart
+++ b/pkg/docgen/test/inherited_comments_test.dart
@@ -28,7 +28,7 @@ void main() {
     });
 
     schedule(() {
-      var path = p.join(d.defaultRoot, 'docs', 'dart:core.Set.json');
+      var path = p.join(d.defaultRoot, 'docs', 'dart-core.Set.json');
       var dartCoreSetJson = new File(path).readAsStringSync();
 
       var dartCoreSet = JSON.decode(dartCoreSetJson) as Map<String, dynamic>;