Support subclassing Observable with non-class constructor functions.

apollographql/apollo-client#7640 (comment)

module.js

@@ -1 +1,26 @@
-export { Observable } from "zen-observable/esm.js";
+import { Observable } from "zen-observable/esm.js";
+
+// When a non-native class constructor function attempts to subclass
+// Observable, compilers like Babel and TypeScript may compile the
+// super(subscriber) expression to Observable.call(this, subscriber) or
+// Observable.apply(this, arguments). Calling a native class constructor
+// in this way is forbidden by the ECMAScript specification, but we can
+// preserve backwards compatibility by overriding the Observable.call and
+// Observable.apply methods with an implementation that calls
+// Reflect.construct instead, when available.
+
+Observable.call = function (obs, sub) {
+  return construct(this, obs, sub);
+};
+
+Observable.apply = function (obs, args) {
+  return construct(this, obs, args[0]);
+};
+
+function construct(Super, self, subscriber) {
+  return typeof Reflect === 'object'
+    ? Reflect.construct(Super, [subscriber], self.constructor)
+    : Function.prototype.call.call(Super, self, subscriber) || self;
+}
+
+export { Observable }