remove all 'instanceof GraphQLSchema' checks

Author: Jonas Helfer

src/execution/execute.js

@@ -117,11 +117,6 @@ export function execute(
   operationName?: ?string
 ): Promise<ExecutionResult> {
   invariant(schema, 'Must provide schema');
-  invariant(
-    schema instanceof GraphQLSchema,
-    'Schema must be an instance of GraphQLSchema. Also ensure that there are ' +
-    'not multiple versions of GraphQL installed in your node_modules directory.'
-  );
 
   // If a valid context cannot be created due to incorrect arguments,
   // this will throw an error.

src/utilities/extendSchema.js

@@ -94,10 +94,6 @@ export function extendSchema(
   schema: GraphQLSchema,
   documentAST: Document
 ): GraphQLSchema {
-  invariant(
-    schema instanceof GraphQLSchema,
-    'Must provide valid GraphQLSchema'
-  );
 
   invariant(
     documentAST && documentAST.kind === DOCUMENT,

src/validation/validate.js

@@ -53,11 +53,6 @@ export function validate(
 ): Array<GraphQLError> {
   invariant(schema, 'Must provide schema');
   invariant(ast, 'Must provide document');
-  invariant(
-    schema instanceof GraphQLSchema,
-    'Schema must be an instance of GraphQLSchema. Also ensure that there are ' +
-    'not multiple versions of GraphQL installed in your node_modules directory.'
-  );
   const typeInfo = new TypeInfo(schema);
   return visitUsingRules(schema, typeInfo, ast, rules || specifiedRules);
 }