Print name of previous test when a browser test times out.

tools/testing/dart/browser_controller.dart

Index: tools/testing/dart/browser_controller.dart
diff --git a/tools/testing/dart/browser_controller.dart b/tools/testing/dart/browser_controller.dart
index 6e375499bf2f37690c0ecd93ce40d79157f0fb99..27765697ea3b74db45fe86f46381cd51199c2df6 100644
--- a/tools/testing/dart/browser_controller.dart
+++ b/tools/testing/dart/browser_controller.dart
@@ -1092,6 +1092,9 @@ class BrowserTestRunner {
       if (status.currentTest.lastKnownMessage.length > 0) {
         lastKnownMessage = status.currentTest.lastKnownMessage;
       }
+      if (status.lastTest != null) {
+        lastKnownMessage += '\nPrevious test was ${status.lastTest.url}';
+      }
       // Wait until the browser is closed before reporting the test as timeout.
       // This will enable us to capture stdout/stderr from the browser
       // (which might provide us with information about what went wrong).