[ddc] use snapshot when running worker test.

The snapshot is already available in the build directory, this makes the
test save up 11 times all the parse and warmup costs of loading the
compiler in the VM.

In aggregate, this made the test run in 5 seconds instead of 2 and half
minutes on my local desktop.

Change-Id: Ic0f1ef030b06467999fd0178b599de697f8ccc94
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/264730
Commit-Queue: Sigmund Cherem <sigmund@google.com>
Reviewed-by: Nate Bosch <nbosch@google.com>

pkg/dev_compiler/test/worker/worker_test.dart

@@ -12,18 +12,19 @@ import 'package:test/test.dart';
 
 Directory tmp = Directory.systemTemp.createTempSync('ddc_worker_test');
 File file(String path) => File.fromUri(tmp.uri.resolve(path));
+String _resolvePath(String executableRelativePath) {
+  return Uri.file(Platform.resolvedExecutable)
+      .resolve(executableRelativePath)
+      .toFilePath();
+}
 
 void main() {
   var baseArgs = <String>[];
   final executableArgs = <String>[
-    Platform.script
-        .resolve('../../bin/dartdevc.dart')
-        .toFilePath(windows: Platform.isWindows),
+    _resolvePath('gen/dartdevc.dart.snapshot'),
     '--sound-null-safety',
     '--dart-sdk-summary',
-    Uri.file(Platform.resolvedExecutable, windows: Platform.isWindows)
-        .resolve('ddc_outline_sound.dill')
-        .path
+    _resolvePath('ddc_outline_sound.dill'),
   ];
   group('DDC: Hello World', () {
     final argsFile = file('hello_world.args');