pkg/docgen: sort items in index.txt

pkg/docgen/test/generate_json_test.dart

 import 'dart:convert';
 import 'dart:io';
 
 import 'package:path/path.dart' as p;
 import 'package:scheduled_test/descriptor.dart' as d;
 import 'package:scheduled_test/scheduled_test.dart';
 
 import 'util.dart';
 import '../lib/docgen.dart' as dg;
 
@@ skipping 18 matching lines @@
 
   test('json output', () {
     schedule(() {
       var codeDir = getMultiLibraryCodePath();
       expect(FileSystemEntity.isDirectorySync(codeDir), isTrue);
       return dg.docgen(['$codeDir/'], out: p.join(d.defaultRoot, 'docs'));
     });
 
     d.dir('docs', [
         d.matcherFile('index.json', _isJsonMap),
-        d.matcherFile('index.txt', isNotNull),
+        d.matcherFile('index.txt', _hasSortedLines),
         d.matcherFile('library_list.json', _isJsonMap),
         d.matcherFile('testLib-bar.C.json', _isJsonMap),
         d.matcherFile('testLib-bar.json', _isJsonMap),
         d.matcherFile('testLib.A.json', _isJsonMap),
         d.matcherFile('testLib.B.json', _isJsonMap),
         d.matcherFile('testLib.C.json', _isJsonMap),
         d.matcherFile('testLib.json', _isJsonMap),
         d.matcherFile('testLib2-foo.B.json', _isJsonMap),
         d.matcherFile('testLib2-foo.json', _isJsonMap)
     ]).validate();
+
   });
 }
 
+final Matcher _hasSortedLines = predicate((String input) {
+  var lines = new LineSplitter().convert(input);
+
+  var sortedLines = new List.from(lines)..sort();
+
+  var orderedMatcher = orderedEquals(sortedLines);
+  return orderedMatcher.matches(lines, {});
+}, 'String has sorted lines');
+
 final Matcher _isJsonMap = predicate((input) {
   try {
     return JSON.decode(input) is Map;
   } catch (e) {
     return false;
   }
 }, 'Output is JSON encoded Map');