Fix missing schema's astNode when schema was defined inside extension (#1698)

Author: IvanGoncharov

src/utilities/__tests__/extendSchema-test.js

@@ -1165,14 +1165,15 @@ describe('extendSchema', () => {
       });
       expect(schema.getQueryType()).to.equal(undefined);
 
-      const ast = parse(`
+      const extensionSDL = dedent`
         schema @foo {
           query: Foo
-        }
-      `);
-      schema = extendSchema(schema, ast);
+        }`;
+      schema = extendSchema(schema, parse(extensionSDL));
+
       const queryType = schema.getQueryType();
       expect(queryType).to.include({ name: 'Foo' });
+      expect(print(schema.astNode)).to.equal(extensionSDL);
     });
 
     it('adds new root types via schema extension', () => {

src/utilities/extendSchema.js

@@ -206,7 +206,7 @@ export function extendSchema(
 
     types: objectValues(typeMap),
     directives: getMergedDirectives(),
-    astNode: schemaConfig.astNode,
+    astNode: schemaDef || schemaConfig.astNode,
     extensionASTNodes: schemaConfig.extensionASTNodes.concat(schemaExts),
     allowedLegacyNames,
   });