Fix the list of browsers on non-Mac platforms.

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 68a00a2b0c471fc026d3944563ddafa93be564fc..910f98f7b4d29e6fd462e9131d41f1987ba23061 100644
--- a/test/runner/runner_test.dart
+++ b/test/runner/runner_test.dart
@@ -38,6 +38,10 @@ void main() {
 
 final _defaultConcurrency = math.max(1, Platform.numberOfProcessors ~/ 2);
 
+final _browsers =
+    "[vm (default), dartium, content-shell, chrome, phantomjs, firefox" +
+        (Platform.isMacOS ? ", safari" : "") + "]";
+
 final _usage = """
 Usage: pub run test:test [files or directories...]
 
@@ -48,7 +52,7 @@ Usage: pub run test:test [files or directories...]
 
 -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.
-                               [vm (default), dartium, content-shell, chrome, phantomjs, firefox, safari]
+                               $_browsers
 
 -j, --concurrency=<threads>    The number of concurrent test suites run.
                                (defaults to $_defaultConcurrency)