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

test/runner/runner_test.dart

Index: test/runner/runner_test.dart
diff --git a/test/runner/runner_test.dart b/test/runner/runner_test.dart
index 389c97180ab3257d3669e41e54c4340733107481..8817b8b17001f2dff58a35dd28ed0ddb58a6b988 100644
--- a/test/runner/runner_test.dart
+++ b/test/runner/runner_test.dart
@@ -58,6 +58,10 @@ Usage: pub run test:test [files or directories...]
                                (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.