Use a more accurate heuristic for identifying ES6 classes (#49)

src/manager.ts

@@ -92,7 +92,7 @@ export class Manager extends ManagerBase implements IWidgetManager {
         // The ES6 classes cannot be subclassed via Backbone's extend that some
         // code uses, so if the export looks like a class use swizzle to make it
         // extensible.
-        if (value.prototype) {
+        if (isES6Class(value)) {
           value = swizzle(value);
         }
         module[key] = value;
@@ -107,7 +107,7 @@ export class Manager extends ManagerBase implements IWidgetManager {
         // The ES6 classes cannot be subclassed via Backbone's extend that some
         // code uses, so if the export looks like a class use swizzle to make it
         // extensible.
-        if (value.prototype) {
+        if (isES6Class(value)) {
           value = swizzle(value);
         }
         module[key] = value;
@@ -248,6 +248,10 @@ export class Manager extends ManagerBase implements IWidgetManager {
   }
 }
 
+function isES6Class(value: unknown): boolean {
+  return typeof value === 'function' && value.toString().startsWith('class ');
+}
+
 class ClassicComm implements IClassicComm {
   constructor(private readonly id: string, private readonly comm: IComm) {}
   get target_name() {