Extend Future.then inference to subclasses

pkg/analyzer/lib/src/generated/type_system.dart

 // Copyright (c) 2015, 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 analyzer.src.generated.type_system;
 
 import 'dart:collection';
 import 'dart:math' as math;
 
 import 'package:analyzer/dart/ast/token.dart' show TokenType;
@@ skipping 1648 matching lines @@
   @override
   TypeImpl pruned(List<FunctionTypeAliasElement> prune) =>
       throw new UnsupportedError('Future unions are not substituted');
 
   @override
   DartType substitute2(List<DartType> args, List<DartType> params,
           [List<FunctionTypeAliasElement> prune]) =>
       throw new UnsupportedError('Future unions are not used in typedefs');
 
   /**
-   * Creates a union of `T | Future<T>`, unless `T` is already a future-union,
-   * in which case it simply returns `T`
+   * Creates a union of `flatten(T) | Future<flatten(T)>`, unless `T` is
+   * already a future-union, in which case it simply returns `T`
    */
   static DartType from(
       DartType type, TypeProvider provider, TypeSystem system) {
     if (type is FutureUnionType) {
       return type;
     }
     return new FutureUnionType(type, provider, system);
   }
 }
 
 bool _isBottom(DartType t, {bool dynamicIsBottom: false}) {
   return (t.isDynamic && dynamicIsBottom) || t.isBottom;
 }
 
 bool _isTop(DartType t, {bool dynamicIsBottom: false}) {
   // TODO(leafp): Document the rules in play here
   return (t.isDynamic && !dynamicIsBottom) || t.isObject;
 }