Supported "in" operator for all SkylarkIndexable objects.

--
MOS_MIGRATED_REVID=135483694
diff --git a/src/main/java/com/google/devtools/build/lib/rules/AliasConfiguredTarget.java b/src/main/java/com/google/devtools/build/lib/rules/AliasConfiguredTarget.java
index 347c728..1c6eebd 100644
--- a/src/main/java/com/google/devtools/build/lib/rules/AliasConfiguredTarget.java
+++ b/src/main/java/com/google/devtools/build/lib/rules/AliasConfiguredTarget.java
@@ -96,6 +96,11 @@
   }
 
   @Override
+  public boolean containsKey(Object key, Location loc) throws EvalException {
+    return actual != null && actual.containsKey(key, loc);
+  }
+
+  @Override
   public Target getTarget() {
     return actual == null ? null : actual.getTarget();
   }