Fix a bug in the handling of Phi nodes.

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

Index: sdk/lib/_internal/compiler/implementation/inferrer/concrete_types_inferrer.dart
diff --git a/sdk/lib/_internal/compiler/implementation/inferrer/concrete_types_inferrer.dart b/sdk/lib/_internal/compiler/implementation/inferrer/concrete_types_inferrer.dart
index 4d9a704cbc44e83a08ff114772329a6dcdc36e76..129f49c1bfc0cdd84542d05fa2f39677b810b73f 100644
--- a/sdk/lib/_internal/compiler/implementation/inferrer/concrete_types_inferrer.dart
+++ b/sdk/lib/_internal/compiler/implementation/inferrer/concrete_types_inferrer.dart
@@ -475,6 +475,12 @@ class ConcreteTypeSystem extends TypeSystem<ConcreteType> {
   }
 
   @override
+  ConcreteType allocateLoopPhi(Node node, Local variable,
+                               ConcreteType inputType) {
+    return inputType;
+  }
+
+  @override
   ConcreteType computeLUB(ConcreteType firstType, ConcreteType secondType) {
     if (firstType == null) {
       return secondType;