Reapply "Create an app snapshot of the Dart front end."

utils/kernel-service/kernel-service.dart

 // Copyright (c) 2016, 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 runtime.tools.kernel_service;
 
 // This is an interface to the Dart Kernel parser and Kernel binary generator.
 //
 // It is used by the kernel-isolate to load Dart source code and generate
 // Kernel binary format.
 //
@@ skipping 187 matching lines @@
   } else {
     // See loader.cc for the code that handles these replies.
     if (result is CompilationOk) {
       port.send([tag, inputFileUrl, inputFileUrl, null, result]);
     } else {
       port.send([-tag, inputFileUrl, inputFileUrl, null, result.errorString]);
     }
   }
 }
 
-main() => new RawReceivePort()..handler = _processLoadRequest;
+// This entry point is used when running in the kernel isolate.
+start() => new RawReceivePort()..handler = _processLoadRequest;
+
+// This entry point is used when creating an app snapshot. The argument provides
+// a script to compile to warm-up generated code.
+main(args) {
+  // TODO(28532): Enable on Windows.
+  if (Platform.isWindows) return;
+
+  var tag = 1;
+  var scriptUri = args[0];
+  var responsePort = new RawReceivePort();
+  responsePort.handler = (response) {
+    if (response[0] == tag) {
+      // Success.
+      responsePort.close();
+    } else if (response[0] == -tag) {
+      // Compilation error.
+      throw response[4];
+    } else {
+      throw "Unexpected response: $response";
+    }
+  };
+  var request = [tag, responsePort.sendPort, scriptUri];
+  _processLoadRequest(request);
+}
 
 // This duplicates functionality from the Loader which we can't easily
 // access from here.
 Uri _findPackagesFile(Uri base) async {
   var dir = new File.fromUri(base).parent;
   while (true) {
     final packagesFile = dir.uri.resolve(".packages");
     if (await new File.fromUri(packagesFile).exists()) {
       return packagesFile;
     }
-    if (dir.parent == dir) {

[rmacnak, 2017/01/31 01:53:37]

This is always true. dart.io.Directory are only equal if they are identical, not
if they have the same path/inode/etc.

[Vyacheslav Egorov (Google), 2017/01/31 06:18:02]

This code was copied from the Loader code so it has the same bug. The reason why
this code is written like this is because we have the following comment for the
parent:

  /**
   * The directory containing [this].  If [this] is a root
   * directory, returns [this].
   */

But implementation does not actually follow the comment :-/

[Vyacheslav Egorov (Google), 2017/01/31 07:32:22]

Ah, disregard this comment. Loader does not have this bug --- I changed it when
I copied it because I read the description of the .parent and thought that I am
improving it, but it turns out that loader code was working around the
documentation problem. 

+    if (dir.parent.path == dir.path) {
       break;
     }
     dir = dir.parent;
   }
   return null;
 }