Refactor extension type template tests to use test_reflective_loader (#3676)

Author: srawlins

lib/src/model_utils.dart

@@ -9,12 +9,16 @@ import 'package:analyzer/file_system/file_system.dart';
 import 'package:dartdoc/src/failure.dart';
 import 'package:dartdoc/src/model/model.dart';
 import 'package:glob/glob.dart';
+import 'package:meta/meta.dart';
 import 'package:path/path.dart' as path;
 
 final _driveLetterMatcher = RegExp(r'^\w:\\');
 
 final Map<String, String> _fileContents = <String, String>{};
 
+@visibleForTesting
+void clearFileContentsCache() => _fileContents.clear();
+
 /// This will handle matching globs, including on Windows.
 ///
 /// On windows, globs are assumed to use absolute Windows paths with drive

test/src/test_descriptor_utils.dart

@@ -5,6 +5,7 @@
 import 'dart:convert';
 
 import 'package:analyzer/file_system/memory_file_system.dart';
+import 'package:dartdoc/src/model_utils.dart';
 import 'package:test_descriptor/test_descriptor.dart' as d;
 import 'package:yaml/yaml.dart' as yaml;
 
@@ -84,6 +85,7 @@ Future<String> createPackage(
       ],
     ),
   ]);
+  clearFileContentsCache();
   if (resourceProvider == null) {
     await packageDir.create();
     return packageDir.io.path;