Fix failure of test_runner_test on Windows.

tools/testing/dart/test_runner.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.
 
 /**
  * Classes and methods for executing tests.
  *
  * This module includes:
  * - Managing parallel execution of tests, including timeout checks.
  * - Evaluating the output of each test as pass/fail/crash/timeout.
@@ skipping 1896 matching lines @@
               } else if (io.Platform.isMacOS) {
                 // Try to print stack traces of the timed out process.
                 // `sample` is a sampling profiler but we ask it sample for 1
                 // second with a 4 second delay between samples so that we only
                 // sample the threads once.
                 executable = '/usr/bin/sample';
                 arguments = ['${process.pid}', '1', '4000', '-mayDie'];
               } else if (io.Platform.isWindows) {
                 bool is_x64 = command.executable.contains("X64") ||
                               command.executable.contains("SIMARM64");
-                executable = configuration['win_sdk_path'] +
-                    "\\Debuggers\\" + (is_x64 ? "x64" : "x86") + "\\cdb.exe";
-                diagnostics.add("Using $executable to print stack traces");
-                arguments = ['-p', '${process.pid}', '-c', '!uniqstack;qd'];
+                var win_sdk_path = configuration['win_sdk_path'];
+                if (win_sdk_path != null) {
+                  executable = win_sdk_path +
+                      "\\Debuggers\\" + (is_x64 ? "x64" : "x86") + "\\cdb.exe";
+                  diagnostics.add("Using $executable to print stack traces");
+                  arguments = ['-p', '${process.pid}', '-c', '!uniqstack;qd'];
+                } else {
+                  diagnostics.add("win_sdk path not found");
+                }
               }
 
               if (executable != null) {
                 try {
                   var result = await io.Process.run(executable, arguments);
                   diagnostics.addAll(result.stdout.split('\n'));
                   diagnostics.addAll(result.stderr.split('\n'));
                 } catch (error) {
                   diagnostics.add("Unable to capture stack traces: $error");
                 }
@@ skipping 1323 matching lines @@
     }
   }
 
   void eventAllTestsDone() {
     for (var listener in _eventListener) {
       listener.allDone();
     }
     _allDone();
   }
 }