[js] Allow disabling the promise manager through a property setter.

Using the property setter will always take precedence over the environment
variable.

#2969

javascript/node/selenium-webdriver/lib/promise.js

@@ -1991,7 +1991,11 @@ class Scheduler {
 }
 
 
+let USE_PROMISE_MANAGER;
 function usePromiseManager() {
+ if (typeof USE_PROMISE_MANAGER !== 'undefined') {
+ return !!USE_PROMISE_MANAGER;
+ }
  return process.env['SELENIUM_PROMISE_MANAGER'] === undefined
  || !/^0|false$/i.test(process.env['SELENIUM_PROMISE_MANAGER']);
 }
@@ -3243,8 +3247,21 @@ module.exports = {
  setDefaultFlow: setDefaultFlow,
  when: when,
 
- /** @return {boolean} Whether the promise manager is enabled. */
+ /**
+ * Indicates whether the promise manager is currently enabled. When disabled,
+ * attempting to use the {@link ControlFlow} or {@link ManagedPromise Promise}
+ * classes will generate an error.
+ *
+ * The promise manager is currently enabled by default, but may be disabled
+ * by setting the environment variable `SELENIUM_PROMISE_MANAGER=0` or by
+ * setting this property to false. Setting this property will always take
+ * precedence ove the use of the environment variable.
+ *
+ * @return {boolean} Whether the promise manager is enabled.
+ * @see <https://github.com/SeleniumHQ/selenium/issues/2969>
+ */
  get USE_PROMISE_MANAGER() { return usePromiseManager(); },
+ set USE_PROMISE_MANAGER(/** boolean */value) { USE_PROMISE_MANAGER = value; },
 
  get LONG_STACK_TRACES() { return LONG_STACK_TRACES; },
  set LONG_STACK_TRACES(v) { LONG_STACK_TRACES = v; },