[build] Ensure kernel service trains on the source.

This is follow-up to https://dart-review.googlesource.com/c/sdk/+/117286.

Fixes dartbug.com/38427.

Change-Id: I8e3227addf459577e5126e9c48d1cb95b170157a
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/117464
Commit-Queue: Alexander Aprelev <[email protected]>
Reviewed-by: Ryan Macnak <[email protected]>
Reviewed-by: Alexander Markov <[email protected]>

Author: aam

pkg/vm/lib/frontend_server.dart

@@ -902,6 +902,11 @@ Future<int> starter(
   }
 
   if (options['train']) {
+    if (options.rest.isEmpty) {
+      throw Exception('Must specify input.dart');
+    }
+
+    final String input = options.rest[0];
     final String sdkRoot = options['sdk-root'];
     final String platform = options['platform'];
     final Directory temp =
@@ -920,8 +925,7 @@ Future<int> starter(
       compiler ??=
           new FrontendCompiler(output, printerFactory: binaryPrinterFactory);
 
-      await compiler.compile(Platform.script.toFilePath(), options,
-          generator: generator);
+      await compiler.compile(input, options, generator: generator);
       compiler.acceptLastDelta();
       await compiler.recompileDelta();
       compiler.acceptLastDelta();

pkg/vm/test/frontend_server_test.dart

@@ -28,7 +28,7 @@ Future<int> main() async {
     final CompilerInterface compiler = new _MockedCompiler();
 
     test('train with mocked compiler completes', () async {
-      await starter(<String>['--train'], compiler: compiler);
+      await starter(<String>['--train', 'foo.dart'], compiler: compiler);
     });
   });
 

utils/kernel-service/BUILD.gn

@@ -29,6 +29,7 @@ kernel_application_snapshot("kernel-service_snapshot") {
   ]
   training_args = [
     "--train",
+
     # Force triple-slashes both on Windows and otherwise.
     # Becomes e.g. file:///full/path/to/file and "file:///C:/full/path/to/file.
     # Without the ', "/"' part, on Linux it would get four slashes.
@@ -62,6 +63,7 @@ application_snapshot("frontend_server") {
     "--train",
     "--sdk-root=$sdk_root/",
     "--platform=$sdk_root/vm_platform_strong.dill",
+    rebase_path(main_dart),
   ]
   output = "$root_out_dir/frontend_server.dart.snapshot"
 }