Downwards inference. This adds support to the resolver for downwards

pkg/analyzer/lib/src/generated/element.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 engine.element;
 
 import 'dart:collection';
 import 'dart:math' show min;
 
 import 'package:analyzer/src/generated/utilities_general.dart';
@@ skipping 7036 matching lines @@
       result.typeArguments =
           typeArguments.map((TypeImpl t) => t.pruned(prune)).toList();
       return result;
     }
   }
 
   @override
   InterfaceTypeImpl substitute2(
       List<DartType> argumentTypes, List<DartType> parameterTypes,
       [List<FunctionTypeAliasElement> prune]) {
-    // Pruned types should only ever result from performing type variable
-    // substitution, and it doesn't make sense to substitute again after
-    // substituting once.
-    assert(this.prunedTypedefs == null);
     if (argumentTypes.length != parameterTypes.length) {
       throw new IllegalArgumentException(
           "argumentTypes.length (${argumentTypes.length}) != parameterTypes.length (${parameterTypes.length})");
     }
     if (argumentTypes.length == 0 || typeArguments.length == 0) {
       return this.pruned(prune);
     }
     List<DartType> newTypeArguments = TypeImpl.substitute(
         typeArguments, argumentTypes, parameterTypes, prune);
     if (JavaArrays.equals(newTypeArguments, typeArguments)) {
@@ skipping 4074 matching lines @@
 
   @override
   void visitElement(Element element) {
     int offset = element.nameOffset;
     if (offset != -1) {
       map[offset] = element;
     }
     super.visitElement(element);
   }
 }