Revert "Train frontend_server snapshot (#4512)" (flutter/engine#4556)

This reverts commit d7e7abbcc2 as it
breaks Windows engine buildbot.

engine/src/flutter/frontend_server/BUILD.gn

@@ -7,8 +7,7 @@ import("//third_party/dart/utils/application_snapshot.gni")
 application_snapshot("frontend_server") {
   main_dart = "bin/starter.dart"
   dot_packages = rebase_path(".packages")
-  flutter_patched_sdk = rebase_path("$root_out_dir/flutter_patched_sdk")
-  training_args = [ "--train", "--sdk-root=$flutter_patched_sdk" ]
+  training_args = [ "--train" ]
 
   frontend_server_files = exec_script("//third_party/dart/tools/list_dart_files.py",
      [ "absolute", rebase_path("."), ], "list lines")

engine/src/flutter/frontend_server/lib/server.dart

@@ -261,17 +261,6 @@ Future<int> starter(
   }
 
   if (options['train']) {
-    final String sdkRoot = options['sdk-root'];
-    options = _argParser.parse(<String>['--incremental', '--sdk-root=$sdkRoot']);
-    compiler ??= new _FrontendCompiler(output, printerFactory: binaryPrinterFactory);
-    await compiler.compile(Platform.script.toFilePath(), options, generator: generator);
-    compiler.acceptLastDelta();
-    await compiler.recompileDelta();
-    compiler.acceptLastDelta();
-    await compiler.recompileDelta();
-    compiler.acceptLastDelta();
-    await compiler.recompileDelta();
-    compiler.acceptLastDelta();
     return 0;
   }
 

engine/src/flutter/frontend_server/test/server_test.dart

@@ -26,10 +26,8 @@ class _MockedBinaryPrinter extends Mock implements BinaryPrinter {}
 
 Future<int> main() async {
   group('basic', () {
-    final CompilerInterface compiler = new _MockedCompiler();
-
-    test('train with mocked compiler completes', () async {
-      expect(await starter(<String>['--train'], compiler: compiler), equals(0));
+    test('train completes', () async {
+      expect(await starter(<String>['--train']), equals(0));
     });
   });