Revert "Remove all 'instanceof GraphQLSchema' checks"

src/execution/execute.js

@@ -117,6 +117,11 @@ 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

@@ -94,6 +94,10 @@ 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

@@ -53,6 +53,11 @@ 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);
 }