pkg/docgen: ensure parameter maps keep their key order

pkg/docgen/test/constant_argument_test.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library docgen.test.typedef;
 
 import 'dart:convert';
 import 'dart:io';
 
 import 'package:path/path.dart' as p;
@@ skipping 24 matching lines @@
 
       //
       // Validate function doc references
       //
       var functionDef =
           testLibBar['functions']['methods']['positionalDefaultValues']
           as Map<String, dynamic>;
 
       var params = functionDef['parameters'] as Map<String, dynamic>;
 
+      expect(params.keys, orderedEquals(_PARAM_NAME_ORDER),
+          reason: 'parameter order  must be maintained');
+
       var vals = {};
       params.forEach((paramName, paramHash) {
         expect(_PARAM_VALUES, contains(paramName));
         expect(paramHash['value'], _PARAM_VALUES[paramName],
             reason: 'Value for $paramName should match expected');
       });
     });
   });
 }
 
 final _PARAM_VALUES = {
+  "intConst": "42",
   "boolConst": "true",
-  "intConst": "42",
   "listConst": '[true, 42, "Shanna", null, 3.14, []]',
+  "stringConst": "\"Shanna\"",
   "mapConst": startsWith("Map"),
-  "emptyMap": '{}',
-  "stringConst": "\"Shanna\""
+  "emptyMap": '{}'
 };
+
+const _PARAM_NAME_ORDER = const [
+  "intConst",
+  "boolConst",
+  "listConst",
+  "stringConst",
+  "mapConst",
+  "emptyMap"
+];