Run frontend server from temp directory to force absolute URIs in Kernel file. (#4394)

* Run frontend server from temp directory to force absolute URIs in Kernel file.

* Create special directory to cwd into to avoid conflicts with app sources in temp folder

* Make variable final

frontend_server/lib/server.dart

@@ -258,6 +258,13 @@ Future<int> starter(List<String> args, {
   compiler ??= new _FrontendCompiler(output, printerFactory: binaryPrinterFactory);
   input ??= stdin;
 
+  // Has to be a directory, that won't have any of the compiled application
+  // sources, so that no relative paths could show up in the kernel file.
+  Directory.current = Directory.systemTemp;
+  final Directory workingDirectory = new Directory("flutter_frontend_server");
+  workingDirectory.createSync();
+  Directory.current = workingDirectory;
+
   if (options.rest.isNotEmpty) {
     await compiler.compile(options.rest[0], options, generator: generator);
     return 0;