[frontend_server] output dill during initial compilation to JavaScript

This will allow the tool to use initialize from dill to speedup subsequent runs. In local testing, adding this and initialize from dill sped up the initial compile of the flutter_gallery from 14 seconds to 7 seconds.

https://github.com/flutter/flutter/issues/50616

Change-Id: Ie6bf08ec547c903da75177d892fdbb36a8924d0b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/139313
Reviewed-by: Alexander Aprelev <aam@google.com>
Commit-Queue: Jonah Williams <jonahwilliams@google.com>

pkg/frontend_server/lib/frontend_server.dart

@@ -521,11 +521,10 @@ class FrontendCompiler implements CompilerInterface {
       if (_compilerOptions.target.name == 'dartdevc') {
         await writeJavascriptBundle(
             results, _kernelBinaryFilename, options['filesystem-scheme']);
-      } else {
-        await writeDillFile(results, _kernelBinaryFilename,
-            filterExternal: importDill != null,
-            incrementalSerializer: incrementalSerializer);
       }
+      await writeDillFile(results, _kernelBinaryFilename,
+          filterExternal: importDill != null,
+          incrementalSerializer: incrementalSerializer);
 
       _outputStream.writeln(boundaryKey);
       await _outputDependenciesDelta(results.compiledSources);