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

utils/kernel-service/kernel-service.dart

Index: utils/kernel-service/kernel-service.dart
diff --git a/runtime/tools/kernel-service.dart b/utils/kernel-service/kernel-service.dart
similarity index 88%
rename from runtime/tools/kernel-service.dart
rename to utils/kernel-service/kernel-service.dart
index b1b441db2ed5e7d1f6efe595150f09d973bb1c93..2db4d32566e9f190adcdaf5ab738a0986ec90648 100644
--- a/runtime/tools/kernel-service.dart
+++ b/utils/kernel-service/kernel-service.dart
@@ -205,7 +205,32 @@ Future _processLoadRequest(request) async {
   }
 }
 
-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.
@@ -216,7 +241,7 @@ Uri _findPackagesFile(Uri base) async {
     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;