add --x-package-warnings-prefix option to dartanalyzer

pkg/analyzer_cli/test/data/package_prefix/main.dart

Index: pkg/analyzer_cli/test/data/package_prefix/main.dart
diff --git a/pkg/analyzer_cli/test/data/package_prefix/main.dart b/pkg/analyzer_cli/test/data/package_prefix/main.dart
new file mode 100644
index 0000000000000000000000000000000000000000..4f1f3134b371559743a021e200156b383d22cfe2
--- /dev/null
+++ b/pkg/analyzer_cli/test/data/package_prefix/main.dart
@@ -0,0 +1,14 @@
+// Test data for package_prefix_test.dart
+//
+// To test manually:
+//
+// This should show a hint about an unused import in foo:
+// dart ../../../bin/analyzer.dart --packages=packagelist \
+//   --x-package-warnings-prefix=f main.dart
+
+import "package:foo/foo.dart";
+import "package:bar/bar.dart";
+
+main() {
+  print("$foo$bar");
+}