instanceOf: disable dev instanceOf checks if NODE_ENV not set

development behavior should be opt in, not opt out

graphql#4075 (comment)

.mocharc.yml

@@ -5,3 +5,5 @@ extension:
   - ts
 node-option:
   - 'loader=ts-node/esm/transpile-only'
+require:
+  - './resources/mochaFixture.ts'

resources/mochaFixture.ts

@@ -0,0 +1,3 @@
+export function mochaGlobalSetup() {
+  process.env.NODE_ENV = 'development';
+}

src/jsutils/instanceOf.ts

@@ -1,10 +1,10 @@
 import { inspect } from './inspect.js';
 
 /* c8 ignore next 3 */
-const isProduction =
+const isDevelopment =
   globalThis.process != null &&
   // eslint-disable-next-line no-undef
-  process.env.NODE_ENV === 'production';
+  process.env.NODE_ENV === 'development';
 
 /**
  * A replacement for instanceof which includes an error warning when multi-realm
@@ -15,11 +15,8 @@ const isProduction =
 export const instanceOf: (value: unknown, constructor: Constructor) => boolean =
   /* c8 ignore next 6 */
   // FIXME: https://github.com/graphql/graphql-js/issues/2317
-  isProduction
+  isDevelopment
     ? function instanceOf(value: unknown, constructor: Constructor): boolean {
-        return value instanceof constructor;
-      }
-    : function instanceOf(value: unknown, constructor: Constructor): boolean {
         if (value instanceof constructor) {
           return true;
         }
@@ -50,6 +47,9 @@ spurious results.`,
           }
         }
         return false;
+      }
+    : function instanceOf(value: unknown, constructor: Constructor): boolean {
+        return value instanceof constructor;
       };
 
 interface Constructor extends Function {