compile and ship the front_end blaze worker entrypoint as a snapshot

Bug:
Change-Id: Id8611e484df73a1ffb36baca5c529327bbb336cb
Reviewed-on: https://dart-review.googlesource.com/24560
Reviewed-by: Sigmund Cherem <sigmund@google.com>
Commit-Queue: Jake Macdonald <jakemac@google.com>

pkg/front_end/tool/bazel/worker.dart

@@ -73,6 +73,7 @@ List<String> preprocessArgs(List<String> args) {
 
 /// An [ArgParser] for generating kernel summaries.
 final summaryArgsParser = new ArgParser()
+  ..addFlag('help', negatable: false)
   ..addOption('dart-sdk-summary')
   ..addOption('input-summary', allowMultiple: true)
   ..addOption('multi-root', allowMultiple: true)
@@ -92,6 +93,12 @@ Future<bool> computeSummary(List<String> args,
     {bool isWorker: false, StringBuffer outputBuffer}) async {
   bool succeeded = true;
   var parsedArgs = summaryArgsParser.parse(args);
+
+  if (parsedArgs['help']) {
+    print(summaryArgsParser.usage);
+    exit(0);
+  }
+
   var options = new CompilerOptions()
     ..packagesFileUri = Uri.parse(parsedArgs['packages-file'])
     ..inputSummaries = parsedArgs['input-summary'].map(Uri.parse).toList()

sdk/BUILD.gn

@@ -44,6 +44,7 @@ declare_args() {
 # ........dartfmt.dart.snapshot
 # ........dartdevc.dart.snapshot
 # ........dartdevk.dart.snapshot
+# ........front_end_worker.dart.snapshot
 # ........pub.dart.snapshot
 # ........utils_wrapper.dart.snapshot
 #.........resources/
@@ -161,6 +162,10 @@ _full_sdk_snapshots = [
     "dartfmt",
     "../utils/dartfmt",
   ],
+  [
+    "front_end_worker",
+    "../utils/front_end:front_end_worker",
+  ],
   [
     "pub",
     "../utils/pub",

utils/front_end/BUILD.gn

@@ -0,0 +1,10 @@
+# Copyright (c) 2017, 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.
+
+import("../application_snapshot.gni")
+
+application_snapshot("front_end_worker") {
+  main_dart = "../../pkg/front_end/tool/bazel/worker.dart"
+  training_args = [ "--help" ]
+}