allow --dart-sdk with --persisent_worker

pkg/analyzer_cli/lib/src/options.dart

Index: pkg/analyzer_cli/lib/src/options.dart
diff --git a/pkg/analyzer_cli/lib/src/options.dart b/pkg/analyzer_cli/lib/src/options.dart
index a8fec7965abec4bf7ab191b19d0b34a273e2d964..fc778f471d57a7d149805238dd19f4bfc0b35f65 100644
--- a/pkg/analyzer_cli/lib/src/options.dart
+++ b/pkg/analyzer_cli/lib/src/options.dart
@@ -465,9 +465,20 @@ class CommandLineOptions {
 
       // Persistent worker.
       if (args.contains('--persistent_worker')) {
-        if (args.length != 2 || !args.contains('--build-mode')) {
+        bool validArgs;
+        if (!args.contains('--build-mode')) {
+          validArgs = false;
+        } else if (args.length == 2) {
+          validArgs = true;
+        } else if (args.length == 4 && args.contains('--dart-sdk')) {
+          validArgs = true;
+        } else {
+          validArgs = false;
+        }
+        if (!validArgs) {
           printAndFail('The --persistent_worker flag should be used with and '
-              'only with the --build-mode flag.');
+              'only with the --build-mode flag, and possibly the --dart-sdk '
+              'option. Got: $args');
           return null; // Only reachable in testing.
         }
         return new CommandLineOptions._fromArgs(results, definedVariables);