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

pkg/analyzer_cli/test/perf_report_test.dart

+// Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+library analyzer_cli.test.perf_report;
+
+import 'dart:convert' show JSON;
+
+import 'package:unittest/unittest.dart';
+
+import 'package:analyzer_cli/src/options.dart';
+import 'package:analyzer_cli/src/perf_report.dart';
+
+main() {
+  test('makePerfReport', () {
+    var options = CommandLineOptions.parse(["somefile.dart"]);
+    var encoded = makePerfReport(1000, 1234, options);
+
+    var json = JSON.decode(encoded);
+    expect(json['totalElapsedTime'], 234);
+    expect(json['options']['sourceFiles'], ["somefile.dart"]);
+  });
+}