Issue 6399. Don't fall back to 'dynamic' when variable has explicit type.

compiler/java/com/google/dart/compiler/type/Types.java

Index: compiler/java/com/google/dart/compiler/type/Types.java
diff --git a/compiler/java/com/google/dart/compiler/type/Types.java b/compiler/java/com/google/dart/compiler/type/Types.java
index 174ca8a5be9af9e4a9135cf62411111235c421d2..3c1d122584c9a97043f2ef9240e00ad37fe9fb4b 100644
--- a/compiler/java/com/google/dart/compiler/type/Types.java
+++ b/compiler/java/com/google/dart/compiler/type/Types.java
@@ -162,7 +162,14 @@ public class Types {
       return interTypes.get(0);
     }
     // create union
-    return new InterfaceTypeUnion(interTypes);
+    return unionTypes(interTypes);
+  }
+
+  /**
+   * @return the {@link InterfaceType} which is union of given ones.
+   */
+  public InterfaceType unionTypes(List<InterfaceType> types) {
+    return new InterfaceTypeUnion(types);
   }
 
   /**