Fix a bug in the handling of Phi nodes.

sdk/lib/_internal/compiler/implementation/inferrer/inferrer_visitor.dart

 // Copyright (c) 2013, 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 inferrer_visitor;
 
 import '../dart2jslib.dart' hide Selector, TypedSelector;
 import '../dart_types.dart';
 import '../elements/elements.dart';
 import '../tree/tree.dart';
@@ skipping 61 matching lines @@
    * Returns a new type that unions [firstInput] and [secondInput].
    */
   T allocateDiamondPhi(T firstInput, T secondInput);
 
   /**
    * Returns a new type for holding the potential types of [element].
    * [inputType] is the first incoming type of the phi.
    */
   T allocatePhi(Node node, Local variable, T inputType);
 
+
+  /**
+   * Returns a new type for holding the potential types of [element].
+   * [inputType] is the first incoming type of the phi. [allocateLoopPhi]
+   * only differs from [allocatePhi] in that it allows the underlying
+   * implementation of [TypeSystem] to differentiate Phi nodes due to loops
+   * from other merging uses.
+   */
+  T allocateLoopPhi(Node node, Local variable, T inputType);
+
   /**
    * Simplies the phi representing [element] and of the type
    * [phiType]. For example, if this phi has one incoming input, an
    * implementation of this method could just return that incoming
    * input type.
    */
   T simplifyPhi(Node node, Local variable, T phiType);
 
   /**
    * Adds [newType] as an input of [phiType].
@@ skipping 506 matching lines @@
     bool changed = false;
     assert(!seenReturnOrThrow);
     handlers.forEach((other) {
       changed = mergeHandler(other) || changed;
     });
     return changed;
   }
 
   void startLoop(Node loop) {
     locals.forEachLocal((Local variable, T type) {
-      T newType = types.allocatePhi(loop, variable, type);
+      T newType = types.allocateLoopPhi(loop, variable, type);
       if (newType != type) {
         locals[variable] = newType;
       }
     });
   }
 
   void endLoop(Node loop) {
     locals.forEachLocal((Local variable, T type) {
       T newType = types.simplifyPhi(loop, variable, type);
       if (newType != type) {
@@ skipping 639 matching lines @@
     return type;
   }
 
   T visitCascade(Cascade node) {
     // Ignore the result of the cascade send and return the type of the cascade
     // receiver.
     visit(node.expression);
     return cascadeReceiverStack.removeLast();
   }
 }