Fix captureJSCallUsage in the face of Error polyfills.

lighthouse-cli/test/fixtures/dobetterweb/dbw_tester.html

@@ -88,6 +88,11 @@ <h2>Do better web tester page</h2>
   <a href="./doesnotexist" target="_blank">internal link is ok</a>
 </template>
 
+<!-- Some websites overwrite the original Error object. The captureJSCallUsage function
+  relies on the native Error object and prepareStackTrace from V8. When overwriting the stack
+  property the e.stack inside the gatherer won't be in the correct format
+  https://github.com/GoogleChrome/lighthouse/issues/1194 -->
+<script>window.Error = function(error) { this.stack = 'stacktrace'; };</script>
 <script>
 function stampTemplate(id, location) {
   const template = document.querySelector('#' + id);

lighthouse-core/gather/driver.js

@@ -675,6 +675,15 @@ class Driver {
     });
   }
 
+  /**
+   * Cache native functions/objects inside window
+   * so we are sure polyfills do not overwrite the native implementations
+   */
+  cacheNatives() {
+    return this.evaluateScriptOnLoad(`window.__nativePromise = Promise;
+        window.__nativeError = Error;`);
+  }
+
   /**
    * Keeps track of calls to a JS function and returns a list of {url, line, col}
    * of the usage. Should be called before page load (in beforePass).
@@ -715,16 +724,17 @@ class Driver {
  * @return {function(...*): *} A wrapper around the original function.
  */
 function captureJSCallUsage(funcRef, set) {
+  const __nativeError = window.__nativeError || Error;
   const originalFunc = funcRef;
-  const originalPrepareStackTrace = Error.prepareStackTrace;
+  const originalPrepareStackTrace = __nativeError.prepareStackTrace;
 
   return function() {
     // Note: this function runs in the context of the page that is being audited.
 
     const args = [...arguments]; // callee's arguments.
 
     // See v8's Stack Trace API https://github.com/v8/v8/wiki/Stack-Trace-API#customizing-stack-traces
-    Error.prepareStackTrace = function(error, structStackTrace) {
+    __nativeError.prepareStackTrace = function(error, structStackTrace) {
       // First frame is the function we injected (the one that just threw).
       // Second, is the actual callsite of the funcRef we're after.
       const callFrame = structStackTrace[1];
@@ -747,12 +757,12 @@ function captureJSCallUsage(funcRef, set) {
       // would be unique.
       return {url, args, line, col, isEval}; // return value is e.stack
     };
-    const e = new Error(`__called ${funcRef.name}__`);
+    const e = new __nativeError(`__called ${funcRef.name}__`);
     set.add(JSON.stringify(e.stack));
 
     // Restore prepareStackTrace so future errors use v8's formatter and not
     // our custom one.
-    Error.prepareStackTrace = originalPrepareStackTrace;
+    __nativeError.prepareStackTrace = originalPrepareStackTrace;
 
     // eslint-disable-next-line no-invalid-this
     return originalFunc.apply(this, arguments);

lighthouse-core/gather/gather-runner.js

@@ -94,7 +94,7 @@ class GatherRunner {
     return driver.assertNoSameOriginServiceWorkerClients(options.url)
       .then(_ => driver.beginEmulation(options.flags))
       .then(_ => driver.enableRuntimeEvents())
-      .then(_ => driver.evaluateScriptOnLoad('window.__nativePromise = Promise;'))
+      .then(_ => driver.cacheNatives())
       .then(_ => driver.cleanAndDisableBrowserCaches())
       .then(_ => driver.clearDataForOrigin(options.url));
   }

lighthouse-core/test/gather/fake-driver.js

@@ -43,6 +43,9 @@ module.exports = {
   },
   cleanAndDisableBrowserCaches() {},
   clearDataForOrigin() {},
+  cacheNatives() {
+    return Promise.resolve();
+  },
   beginTrace() {
     return Promise.resolve();
   },

lighthouse-core/test/gather/gather-runner-test.js

@@ -54,6 +54,9 @@ function getMockedEmulationDriver(emulationFn, netThrottleFn, cpuThrottleFn) {
     assertNoSameOriginServiceWorkerClients() {
       return Promise.resolve();
     }
+    cacheNatives() {
+      return Promise.resolve();
+    }
     cleanAndDisableBrowserCaches() {}
     clearDataForOrigin() {}
   };