Add --x-perf-report flag to the dartanalyzer command

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 d700c4e483c3d178c8ad9dad2b3b1a63b0f3b82b..2bc48b97e5a2faba97f95eb8b605dd382dd0f496 100644
--- a/pkg/analyzer_cli/lib/src/options.dart
+++ b/pkg/analyzer_cli/lib/src/options.dart
@@ -80,6 +80,10 @@ class CommandLineOptions {
   /// The path to a `.packages` configuration file
   final String packageConfigPath;
 
+  /// The path to a file to write a performance log.
+  /// (Or null if not enabled.)
+  final String perfReport;
+
   /// Batch mode (for unit testing)
   final bool shouldBatch;
 
@@ -117,6 +121,7 @@ class CommandLineOptions {
         machineFormat = args['machine'] || args['format'] == 'machine',
         packageConfigPath = args['packages'],
         packageRootPath = args['package-root'],
+        perfReport = args['x-perf-report'],
         shouldBatch = args['batch'],
         showPackageWarnings =
             args['show-package-warnings'] || args['package-warnings'],
@@ -248,6 +253,8 @@ class CommandLineOptions {
           help: 'Show warnings from SDK imports (deprecated).',
           defaultsTo: false,
           negatable: false)
+      ..addOption('x-perf-report',
+          help: 'Writes a performance report to the given file (experimental).')
       ..addFlag('help',
           abbr: 'h',
           help: 'Display this help message.',