Change the type inference for fields in dart2js

lib/compiler/implementation/universe/partial_type_tree.dart

Index: lib/compiler/implementation/universe/partial_type_tree.dart
diff --git a/lib/compiler/implementation/universe/partial_type_tree.dart b/lib/compiler/implementation/universe/partial_type_tree.dart
index 1e8036172fdbfb51ecea8a7b15ba9fd1b0f61010..a8e7bf4497ca9e121f77eaf8144ae682a20448a7 100644
--- a/lib/compiler/implementation/universe/partial_type_tree.dart
+++ b/lib/compiler/implementation/universe/partial_type_tree.dart
@@ -2,8 +2,11 @@
 // 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.
 
-class PartialTypeTree {
+class PartialTypeTree implements Hashable {
 
+  static int _HASH_COUNTER = 0;
+
+  final int _hashCode;
   final Compiler compiler;
   PartialTypeTreeNode root;
 
@@ -22,7 +25,9 @@ class PartialTypeTree {
   Set<ClassElement> unseenInterfaceSubtypes =
       new Set<ClassElement>();
 
-  PartialTypeTree(this.compiler);
+  PartialTypeTree(this.compiler) : _hashCode = ++_HASH_COUNTER;
+
+  hashCode() => _hashCode;
 
   abstract PartialTypeTreeNode newSpecializedNode(ClassElement type);