Fix some flow types

Marks the type resolver as optional, and updates the flow types for mutations
to include info.

Fixes #9.

src/mutation/mutation.js

@@ -17,11 +17,12 @@ import {
 
 import type {
   GraphQLFieldConfig,
-  GraphQLFieldConfigMap
+  GraphQLFieldConfigMap,
+  GraphQLResolveInfo
 } from 'graphql';
 
-type mutationFn = (object: Object) => Object |
-                  (object: Object) => Promise<Object>;
+type mutationFn = (object: Object, info: GraphQLResolveInfo) => Object |
+                  (object: Object, info: GraphQLResolveInfo) => Promise<Object>;
 
 /**
  * A description of a mutation consumable by mutationWithClientMutationId

src/node/node.js

@@ -38,10 +38,14 @@ type typeResolverFn = (object: any) => ?GraphQLObjectType |
  * to map from an underlying object to the concrete GraphQLObjectType it
  * corresponds to, constructs a `Node` interface that objects can implement,
  * and a field config for a `node` root field.
+ *
+ * If the typeResolver is omitted, object resolution on the interface will be
+ * handled with the `isTypeOf` method on object types, as with any GraphQL
+ * interface without a provided `resolveType` method.
  */
 export function nodeDefinitions(
   idFetcher: ((id: string, info: GraphQLResolveInfo) => any),
-  typeResolver: typeResolverFn
+  typeResolver?: ?typeResolverFn
 ): GraphQLNodeDefinitions {
   var nodeInterface = new GraphQLInterfaceType({
     name: 'Node',