Allow client-side schema extensions in optimisticResponse

Summary:
Setting client-side schema extension fields in `optimisticResponse` works fine, but `validateMutation` throws a warning. I've updated `validateMutation` to allow this.

My use case is that I have an "is_saving" flag I'm adding to an object using "Client Schema Extensions". I want to set this flag when a mutation is being performed, and unset it when the mutation is complete. I want to display the state of the mutation in several places in the UI, and using local Relay state seemed like an easier way to do that compared to having to pass some React state through several components. Setting this field using `optimisticResponse` works well because Relay will automatically revert the change after the mutation is complete.

Reviewed By: bennyhobart

Differential Revision: D20228541

fbshipit-source-id: f9ed06739bc0155b29f3da2c28bc727324fc1a16

packages/relay-runtime/mutations/__tests__/validateMutation-test.js

@@ -553,6 +553,68 @@ describe('validateOptimisticResponse', () => {
       variables: null,
       shouldWarn: true,
     },
+    {
+      name: 'Does not log a warning for client-side schema extensions',
+      mutation: generateAndCompile(
+        `
+          extend type Feedback {
+            isSavingLike: Boolean
+          }
+          mutation FeedbackLikeMutation(
+            $input: FeedbackLikeInput
+          ) {
+            feedbackLike(input: $input) {
+              feedback {
+                doesViewerLike
+                isSavingLike
+              }
+            }
+          }
+      `,
+      ).FeedbackLikeMutation,
+      optimisticResponse: {
+        feedbackLike: {
+          feedback: {
+            id: 1,
+            doesViewerLike: true,
+            isSavingLike: true,
+          },
+        },
+      },
+      variables: null,
+      shouldWarn: false,
+    },
+    {
+      name: 'Logs a warning for invalid client-side schema extension fields',
+      mutation: generateAndCompile(
+        `
+          extend type Feedback {
+            isSavingLike: Boolean
+          }
+          mutation FeedbackLikeMutation(
+            $input: FeedbackLikeInput
+          ) {
+            feedbackLike(input: $input) {
+              feedback {
+                doesViewerLike
+                isSavingLike
+              }
+            }
+          }
+      `,
+      ).FeedbackLikeMutation,
+      optimisticResponse: {
+        feedbackLike: {
+          feedback: {
+            id: 1,
+            doesViewerLike: true,
+            someInvalidField: true,
+          },
+        },
+      },
+      variables: null,
+      shouldWarn: true,
+    },
   ].forEach(({name, mutation, optimisticResponse, shouldWarn, variables}) => {
     it(name, () => {
       jest.clearAllMocks();

packages/relay-runtime/mutations/validateMutation.js

@@ -108,6 +108,10 @@ if (__DEV__) {
         });
         return;
       case 'ClientExtension':
+        selection.selections.forEach(subselection => {
+          validateSelection(optimisticResponse, subselection, context);
+        });
+        return;
       case 'ModuleImport':
       case 'LinkedHandle':
       case 'ScalarHandle':