[test_runner] include error stack in window.onerror if available.

According to the [Window.onerror MDN docs][1] an error object may be
provided as a 5th argument. This error object can be used to extract a
stack trace with more context about the error message.

[1]: https://developer.mozilla.org/en-US/docs/Web/API/Window/error_event

With this change, we can get a more precise error on uncuaght exceptions.

This is an example of a before and after on a unit test failing in
ddc-chrome-linux.

Before:

```
  Runtime window.onerror:
  window.onerror called:

  http://127.0.0.1:46235/root_build/gen/utils/ddc/stable/sdk/amd/dart_sdk.js:5082:
  Uncaught Error
```

After:

```
  Runtime window.onerror:
  window.onerror called:

  http://127.0.0.1:46235/root_build/gen/utils/ddc/stable/sdk/amd/dart_sdk.js:5082:
  Uncaught Error
  Error: Expected a value of type 'FutureOr<FileSystem>?', but got one of type 'LegacyJavaScriptObject'
      at Object.throw_ [as throw] (http://127.0.0.1:46235/root_build/gen/utils/ddc/stable/sdk/amd/dart_sdk.js:5082:11)
      at Object.castError (http://127.0.0.1:46235/root_build/gen/utils/ddc/stable/sdk/amd/dart_sdk.js:5041:15)
      at Object.cast [as as] (http://127.0.0.1:46235/root_build/gen/utils/ddc/stable/sdk/amd/dart_sdk.js:5385:19)
      at dart.NullableType.new.as (http://127.0.0.1:46235/root_build/gen/utils/ddc/stable/sdk/amd/dart_sdk.js:7243:60)
      at _AsyncCompleter.new.complete (http://127.0.0.1:46235/root_build/gen/utils/ddc/stable/sdk/amd/dart_sdk.js:31216:28)
      at http://127.0.0.1:46235/root_build/gen/utils/ddc/stable/sdk/amd/dart_sdk.js:99073:19
```

Change-Id: If89c253d8e6edee9b7102269466b40b605951a6c
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/328804
Reviewed-by: Nicholas Shahan <[email protected]>
Commit-Queue: Sigmund Cherem <[email protected]>

Author: sigmundch

pkg/test_runner/lib/src/browser_controller.dart

@@ -1388,12 +1388,15 @@ body div {
         test_completed = false;
       }
 
-      window.onerror = function (message, url, lineNumber) {
+      window.onerror = function (message, url, lineNumber, columnNumber, err) {
+        // Ensure the stack is included in the reported error, if available.
+        if (err && err.stack) {
+          message = message + '\\n' + err.stack;
+        }
         if (url) {
-          reportError(url + ':' + lineNumber + ':' + message);
-        } else {
-          reportError(message);
+          message = url + ':' + lineNumber + ':' + columnNumber + ':' + message;
         }
+        reportError(message);
       }
 
       function reportError(msg) {

pkg/test_runner/lib/src/test_controller.js

@@ -93,10 +93,14 @@ var hadTopLevelError = false;
 
 // Set window onerror to make sure that we catch test harness errors across all
 // browsers.
-window.onerror = function (message, url, lineNumber) {
+window.onerror = function (message, url, lineNumber, columnNumber, err) {
+  // Ensure the stack is included in the reported error, if available.
+  if (err && err.stack) {
+    message = message + '\n' + err.stack;
+  }
   if (url) {
     message = ('window.onerror called: \n\n' +
-        url + ':' + lineNumber + ':\n' + message + '\n\n');
+        url + ':' + lineNumber + ':' + columnNumber + ':\n' + message + '\n\n');
   }
   if (testExpectsGlobalError) {
     testSuppressedGlobalErrors.push({