Print name of previous test when a browser test times out.
Remove timeout multiplier for dart2js browser tests.

BUG=https://github.com/dart-lang/sdk/issues/26060
R=kustermann@google.com

Review URL: https://codereview.chromium.org/1867283002 .
diff --git a/tools/testing/dart/browser_controller.dart b/tools/testing/dart/browser_controller.dart
index 6e37549..2776569 100644
--- a/tools/testing/dart/browser_controller.dart
+++ b/tools/testing/dart/browser_controller.dart
@@ -1092,6 +1092,9 @@
       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).