Remove mutation cache root key. (#745)

Author: sav007

apollo-runtime/src/main/java/com/apollographql/apollo/cache/normalized/CacheKeyResolver.java

@@ -1,9 +1,7 @@
 package com.apollographql.apollo.cache.normalized;
 
-import com.apollographql.apollo.api.ResponseField;
-import com.apollographql.apollo.api.Mutation;
 import com.apollographql.apollo.api.Operation;
-import com.apollographql.apollo.api.Query;
+import com.apollographql.apollo.api.ResponseField;
 
 import java.util.Map;
 
@@ -13,6 +11,8 @@
  * Resolves a cache key for a JSON object.
  */
 public abstract class CacheKeyResolver {
+  private static final CacheKey ROOT_CACHE_KEY = CacheKey.from("QUERY_ROOT");
+
   public static final CacheKeyResolver DEFAULT = new CacheKeyResolver() {
     @Nonnull @Override
     public CacheKey fromFieldRecordSet(@Nonnull ResponseField field, @Nonnull Map<String, Object> recordSet) {
@@ -24,16 +24,10 @@ public CacheKey fromFieldArguments(@Nonnull ResponseField field, @Nonnull Operat
       return CacheKey.NO_KEY;
     }
   };
-  public static final CacheKey QUERY_ROOT_KEY = CacheKey.from("QUERY_ROOT");
-  public static final CacheKey MUTATION_ROOT_KEY = CacheKey.from("MUTATION_ROOT");
 
+  @SuppressWarnings("unused")
   public static CacheKey rootKeyForOperation(@Nonnull Operation operation) {
-    if (operation instanceof Query) {
-      return QUERY_ROOT_KEY;
-    } else if (operation instanceof Mutation) {
-      return MUTATION_ROOT_KEY;
-    }
-    throw new IllegalArgumentException("Unknown operation type.");
+    return ROOT_CACHE_KEY;
   }
 
   @Nonnull public abstract CacheKey fromFieldRecordSet(@Nonnull ResponseField field,