Use "isPromise" instead of testing for 'then' property (#1879)

Author: IvanGoncharov

src/graphql.js

@@ -7,6 +7,7 @@
  * @flow strict
  */
 
+import isPromise from './jsutils/isPromise';
 import { validateSchema } from './type/validate';
 import { parse } from './language/parser';
 import { validate } from './validation/validate';
@@ -175,7 +176,7 @@ export function graphqlSync(
         );
 
   // Assert that the execution was synchronous.
-  if (result.then) {
+  if (isPromise(result)) {
     throw new Error('GraphQL execution failed to complete synchronously.');
   }
 

src/utilities/introspectionFromSchema.js

@@ -8,6 +8,7 @@
  */
 
 import invariant from '../jsutils/invariant';
+import isPromise from '../jsutils/isPromise';
 import { type GraphQLSchema } from '../type/schema';
 import { execute } from '../execution/execute';
 import { parse } from '../language/parser';
@@ -32,6 +33,6 @@ export function introspectionFromSchema(
 ): IntrospectionQuery {
   const queryAST = parse(getIntrospectionQuery(options));
   const result = execute(schema, queryAST);
-  invariant(!result.then && !result.errors && result.data);
+  invariant(!isPromise(result) && !result.errors && result.data);
   return (result.data: any);
 }