Catch errors on top window using uncaughtErrorFilter

Works around safari quirk when running in iframe

test/unit/strict-template-policy.html

@@ -18,15 +18,6 @@
       import { setStrictTemplatePolicy } from '../../lib/utils/settings.js';
       import { Debouncer } from '../../lib/utils/debounce.js';
       setStrictTemplatePolicy(true);
-      // Errors thrown in custom element reactions are not thrown up
-      // the call stack to the dom methods that provoked them, so need
-      // to catch them here and prevent mocha from complaining about them
-      window.addEventListener('error', function(event) {
-        event.stopImmediatePropagation();
-        if (!window.globalError) {
-          window.globalError = event;
-        }
-      });
       // Errors thrown in Polymer's debouncer queue get re-thrown
       // via setTimeout, making them particulary difficult to verify;
       // Wrap debouncer callbacks and store on the globalError to test later
@@ -36,7 +27,9 @@
           try { 
             callback(); 
           } catch(error) {
-            window.globalError = error;
+            if (!window.uncaughtErrorFilter || !window.uncaughtErrorFilter(error)) {
+              throw error;
+            }
           }
         });
       };
@@ -72,26 +65,34 @@
     suite('strictTemplatePolicy', function() {
 
       teardown(function() {
-        window.globalError = null;
+        window.uncaughtErrorFilter = window.top.uncaughtErrorFilter = null;
       });
 
+      // Errors thrown in custom element reactions are not thrown up
+      // the call stack to the dom methods that provoked them, so this
+      // wraps Chai's assert.throws to re-throw uncaught errors
       function assertThrows(fn, re) {
+        let uncaughtError = null;
+        // Catch uncaught errors; note when running in iframe sometimes
+        // Safari errors are thrown on the top window, sometimes not, so
+        // catch in both places
+        window.uncaughtErrorFilter = window.top.uncaughtErrorFilter = function(err) {
+          uncaughtError = err;
+          return true;
+        }
         assert.throws(function() {
           fn();
-          // Throw any errors caught on window
-          if (window.globalError) {
-            throw new Error(window.globalError.message);
+          // Re-throw any uncaughtErrors
+          if (uncaughtError) {
+            throw new Error(uncaughtError.message);
           }
           // Force polyfill reactions and/or async template stamping
           flush();
-          // Throw any add'l errors caught on window
-          if (window.globalError) {
-            throw new Error(window.globalError.message);
+          // Re-throw any uncaughtErrors
+          if (uncaughtError) {
+            throw new Error(uncaughtError.message);
           }
         }, re);
-        if (HTMLTemplateElement.bootstrap) {
-          HTMLTemplateElement.bootstrap();
-        }
       }
 
       test('dom-bind', function() {