Add a --pause-after-load flag for debugging.

test/runner/runner_test.dart

 // Copyright (c) 2015, 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.
 
 @TestOn("vm")
 
 import 'dart:io';
 import 'dart:math' as math;
 
 import 'package:path/path.dart' as p;
@@ skipping 40 matching lines @@
                                Regular expression syntax is supported.
 
 -N, --plain-name               A plain-text substring of the name of the test to run.
 -p, --platform                 The platform(s) on which to run the tests.
                                $_browsers
 
 -j, --concurrency=<threads>    The number of concurrent test suites run.
                                (defaults to $_defaultConcurrency)
 
     --pub-serve=<port>         The port of a pub serve instance serving "test/".
+    --pause-after-load         Pauses for debugging before any tests execute.
+                               Implies --concurrency=1.
+                               Currently only supported for browser tests.
+
 -r, --reporter                 The runner used to print test results.
 
           [compact]            A single line, updated continuously.
           [expanded]           A separate line for each update.
 
     --verbose-trace            Whether to emit stack traces with core library frames.
     --js-trace                 Whether to emit raw JavaScript stack traces for browser tests.
     --[no-]color               Whether to use terminal colors.
                                (auto-detected by default)
 """;
@@ skipping 598 matching lines @@
         d.file("test.dart", _success).create();
 
         var test = runTest(["--plain-name", "no match", "test.dart"]);
         test.stderr.expect(
             consumeThrough(contains('No tests match "no match".')));
         test.shouldExit(exit_codes.data);
       });
     });
   });
 }