Add support for GraphQL TypeExtensionDefinition (#2306)

src/printer-graphql.js

@@ -209,11 +209,17 @@ function genericPrint(path, options, print) {
       ]);
     }
 
+    case "TypeExtensionDefinition": {
+      return concat(["extend ", path.call(print, "definition")]);
+    }
+
     case "ObjectTypeDefinition": {
+      const parent = path.getParentNode();
       return concat([
         "type ",
         path.call(print, "name"),
-        " = {",
+        parent.kind === "TypeExtensionDefinition" ? "" : " =",
+        " {",
         indent(concat([hardline, join(hardline, path.map(print, "fields"))])),
         hardline,
         "}"

tests/graphql_object_type_def/__snapshots__/jsfmt.spec.js.snap

@@ -1,5 +1,16 @@
 // Jest Snapshot v1, https://goo.gl/fbAQLP
 
+exports[`extend.graphql 1`] = `
+extend type Feedback {
+  custom_int: Int
+}
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+extend type Feedback {
+  custom_int: Int
+}
+
+`;
+
 exports[`object_type_def.graphql 1`] = `
 type FeedHomeStories {
   debug_info: String

tests/graphql_object_type_def/extend.graphql

@@ -0,0 +1,3 @@
+extend type Feedback {
+  custom_int: Int
+}