pkg/docgen: fix type warnings

pkg/docgen/lib/src/models/indexable.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.models.indexable;
 
 import 'dart:collection';
 
 import 'package:markdown/markdown.dart' as markdown;
 
@@ skipping 29 matching lines @@
   String get qualifiedName => packagePrefix + ownerPrefix + name;
 
   final TMirror mirror;
   final bool isPrivate;
   /// The comment text pre-resolution. We keep this around because inherited
   /// methods need to resolve links differently from the superclass.
   String unresolvedComment = '';
 
   Indexable(TMirror mirror)
       : this.mirror = mirror,
-        this.isPrivate = isHidden(mirror) {
+        this.isPrivate = isHidden(mirror as DeclarationSourceMirror) {
 
     var mirrorQualifiedName = dart2js_util.qualifiedNameOf(this.mirror);
 
     var map = _mirrorToDocgen.putIfAbsent(mirrorQualifiedName,
         () => new HashMap<String, Indexable>());
 
     var added = false;
     map.putIfAbsent(owner.docName, () {
       added = true;
       return this;
@@ skipping 146 matching lines @@
 /// Why two levels of lookup? Speed, man. Speed.
 final Map<String, Map<String, Indexable>> _mirrorToDocgen =
     new HashMap<String, Map<String, Indexable>>();
 
 Iterable<Indexable> get allIndexables =>
     _mirrorToDocgen.values.expand((map) => map.values);
 
 Map<String, Indexable> lookupIndexableMap(DeclarationMirror mirror) {
   return _mirrorToDocgen[dart2js_util.qualifiedNameOf(mirror)];
 }