Guard property access to __raven_wrapper__

Accessing the `__raven_wrapper__` property in Firefox 48 when run under
Selenium results in the following error:

    Uncaught WebDriverError: Permission denied to access property "__raven_wrapper__" (WARNING: The server did not provide any stacktrace information)
        Command duration or timeout: 10 milliseconds
    Build info: version: '3.0.0-beta2', revision: '2aa21c1', time: '2016-08-02 15:03:28 -0700'
    System info: host: '199-7-164-50', ip: '199.7.164.50', os.name: 'windows', os.arch: 'x86', os.version: '6.2', java.version: '1.8.0_91'
    Driver info: org.openqa.selenium.firefox.FirefoxDriver
    Capabilities [{browserstack.localIdentifier=travis-3840.1, raisesAccessibilityExceptions=false, browserstack.tunnelIdentifier=travis-3840.1, browserstack.selenium.jar.version=3.0.0-beta2, appBuildId=20160726073904, platform=XP, specificationLevel=0, acceptSslCerts=false, browserstack.key=GpeJcf7DBqk3pHnB3PkZ, browser=firefox, browserVersion=48.0, platformVersion=6.2, acceptSslCert=false, XULappId={ec8030f7-c20a-464f-9b0e-13a3a9e97384}, browserName=Firefox, takesScreenshot=true, browser_version=48.0, platformName=Windows_NT, 64bit=true, browserstack.debug=false, rotatable=false, browserstack.ie.noFlash=false, browserstack.user=dhcole, version=48.0, proxy={}, command_id=1, firefox_binary=c:\Program Files (x86)\firefox 48.0\firefox.exe, browserstack.video=true, orig_os=win8, takesElementScreenshot=true, device=desktop, proxy_type=privoxy}]
    Session ID: 8ad81e9d-f345-48a9-b1fe-def338169a10
        at WebDriverError (node_modules/selenium-webdriver/lib/error.js:27:10)
        at Object.checkLegacyResponse (node_modules/selenium-webdriver/lib/error.js:639:15)
        at parseHttpResponse (node_modules/selenium-webdriver/http/index.js:538:13)
        at node_modules/selenium-webdriver/http/index.js:472:11
        at ManagedPromise.invokeCallback_ (node_modules/selenium-webdriver/lib/promise.js:1379:14)
        at TaskQueue.execute_ (node_modules/selenium-webdriver/lib/promise.js:2913:14)
        at TaskQueue.executeNext_ (node_modules/selenium-webdriver/lib/promise.js:2896:21)
        at node_modules/selenium-webdriver/lib/promise.js:2820:25
        at node_modules/selenium-webdriver/lib/promise.js:639:7
    From: Task: WebDriver.executeScript()
        at WebDriver.schedule (node_modules/selenium-webdriver/lib/webdriver.js:377:17)
        at WebDriver.executeScript (node_modules/selenium-webdriver/lib/webdriver.js:526:16)
        at WebDriver.driver.load (test/suites/selenium/_init.js:180:14)
        at test/suites/selenium/_init.js:100:14
        at initDriver (test/suites/selenium/_init.js:192:5)
        at test/suites/selenium/_init.js:147:9
        at node_modules/browserstack-local/lib/Local.js:46:11
        at ChildProcess.exithandler (child_process.js:204:7)
        at maybeClose (internal/child_process.js:829:16)
        at Process.ChildProcess._handle.onexit (internal/child_process.js:211:5)

By wrapping the access, we can avoid the error.

Relates to #495

src/raven.js

@@ -258,18 +258,18 @@ Raven.prototype = {
             if (func.__raven__) {
                 return func;
             }
+
+            // If this has already been wrapped in the past, return that
+            if (func.__raven_wrapper__ ){
+                return func.__raven_wrapper__ ;
+            }
         } catch (e) {
-            // Just accessing the __raven__ prop in some Selenium environments
+            // Just accessing custom props in some Selenium environments
             // can cause a "Permission denied" exception (see raven-js#495).
             // Bail on wrapping and return the function as-is (defers to window.onerror).
             return func;
         }
 
-        // If this has already been wrapped in the past, return that
-        if (func.__raven_wrapper__ ){
-            return func.__raven_wrapper__ ;
-        }
-
         function wrapped() {
             var args = [], i = arguments.length,
                 deep = !options || options && options.deep !== false;
@@ -864,7 +864,11 @@ Raven.prototype = {
                 }, wrappedBuiltIns);
                 fill(proto, 'removeEventListener', function (orig) {
                     return function (evt, fn, capture, secure) {
-                        fn = fn && (fn.__raven_wrapper__ ? fn.__raven_wrapper__  : fn);
+                        try {
+                            fn = fn && (fn.__raven_wrapper__ ? fn.__raven_wrapper__  : fn);
+                        } catch (e) {
+                            // ignore, accessing __raven_wrapper__ will throw in some Selenium environments
+                        }
                         return orig.call(this, evt, fn, capture, secure);
                     };
                 }, wrappedBuiltIns);

test/raven.test.js

@@ -1735,6 +1735,19 @@ describe('Raven (public API)', function() {
             assert.equal(fn, wrapped);
         });
 
+        it('should return input funciton as-is if accessing __raven_wrapper__ prop throws exception', function (){
+            // see raven-js#495
+            var fn = function () {};
+            Object.defineProperty(fn, '__raven_wrapper__', {
+              get: function () {
+                  throw new Error('Permission denied')
+              }
+            });
+            assert.throw(function () { fn.__raven_wrapper__; }, 'Permission denied');
+            var wrapped = Raven.wrap(fn);
+            assert.equal(fn, wrapped);
+        });
+
     });
 
     describe('.context', function() {