Issue 6776. Infer sub-type member only if there is only one candidate.

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

Index: compiler/java/com/google/dart/compiler/type/InterfaceTypeImplementation.java
diff --git a/compiler/java/com/google/dart/compiler/type/InterfaceTypeImplementation.java b/compiler/java/com/google/dart/compiler/type/InterfaceTypeImplementation.java
index fac151a68bf2889a326fc71679b8e1bc3582d9d6..52c63ce6a4698f42a7fb2294e8601b0584f341b4 100644
--- a/compiler/java/com/google/dart/compiler/type/InterfaceTypeImplementation.java
+++ b/compiler/java/com/google/dart/compiler/type/InterfaceTypeImplementation.java
@@ -233,21 +233,32 @@ class InterfaceTypeImplementation extends AbstractType implements InterfaceType
   
   @Override
   public Member lookupSubTypeMember(String name) {
+    Member foundMember = null;
     for (ClassElement subClass : subClasses.keySet()) {
+      // find one or more members in subClass elements
       {
         Element element = subClass.lookupLocalElement(name);
         if (element != null) {
-          return new MemberImplementation(this, element);
+          if (foundMember != null) {
+            return null;
+          }
+          foundMember = new MemberImplementation(this, element);
+          continue;
         }
       }
+      // try to find deeper
       InterfaceType type = subClass.getType();
       if (type != null) {
         Member member = type.lookupSubTypeMember(name);
         if (member != null) {
-          return member;
+          if (foundMember != null) {
+            return null;
+          }
+          foundMember = member;
         }
       }
     }
-    return null;
+    // may be found
+    return foundMember;
   }
 }