Implement input for MRs over child entities

Also throwing in a proof-of-concept MR that I'd like to run in production, and then scrap once the meaty MR is finished (e.g. exploding Recurring billing events into OneTimes).
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=119881471

java/com/google/domain/registry/util/TypeUtils.java

@@ -15,11 +15,13 @@
 package com.google.domain.registry.util;
 
 import static com.google.common.base.Preconditions.checkArgument;
+import static com.google.domain.registry.util.CollectionUtils.difference;
 import static java.lang.reflect.Modifier.isFinal;
 import static java.lang.reflect.Modifier.isStatic;
 
 import com.google.common.base.Predicate;
 import com.google.common.collect.ImmutableMap;
+import com.google.common.collect.ImmutableSet;
 import com.google.common.reflect.TypeToken;
 
 import java.lang.annotation.Annotation;
@@ -88,4 +90,16 @@ public class TypeUtils {
         }
       };
   }
+
+  public static void checkNoInheritanceRelationships(ImmutableSet<Class<?>> resourceClasses) {
+    for (Class<?> resourceClass : resourceClasses) {
+      for (Class<?> potentialSuperclass : difference(resourceClasses, resourceClass)) {
+        checkArgument(
+            !potentialSuperclass.isAssignableFrom(resourceClass),
+            "Cannot specify resource classes with inheritance relationship: %s extends %s",
+            resourceClass,
+            potentialSuperclass);
+      }
+    }
+  }
 }