test.dart: Cope with errors when locating the VS toolchain on Windows.

tools/testing/dart/test_configurations.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library test_configurations;
 
 import "dart:async";
 import 'dart:convert';
 import 'dart:io';
 import "dart:math" as math;
@@ skipping 283 matching lines @@
   }
 
   // Start all the HTTP servers required before starting the process queue.
   if (!serverFutures.isEmpty) {
     await Future.wait(serverFutures);
   }
 
   if (Platform.isWindows) {
     // When running tests on Windows, use cdb from depot_tools to dump
     // stack traces of tests timing out.
-    var text = await new File(VS_TOOLCHAIN_FILE.toNativePath()).readAsString();
-    firstConf['win_sdk_path'] = JSON.decode(text)['win_sdk'];
+    try {
+      var text =
+          await new File(VS_TOOLCHAIN_FILE.toNativePath()).readAsString();
+      firstConf['win_sdk_path'] = JSON.decode(text)['win_sdk'];
+      } on dynamic {
+        // Ignore errors here. If win_sdk is not found, stack trace dumping
+        // for timeouts won't work.
+      }
   }
 
   // [firstConf] is needed here, since the ProcessQueue needs to know the
   // settings of 'noBatch' and 'local_ip'
   new ProcessQueue(
       firstConf,
       maxProcesses,
       maxBrowserProcesses,
       startTime,
       testSuites,
       eventListener,
       allTestsFinished,
       verbose,
       recordingPath,
       recordingOutputPath,
       adbDevicePool);
 }