Merge the Fuchsia frontend_server build script into the new flutter_frontend_server target (#13145)

Author: jason-simmons

flutter_frontend_server/BUILD.gn

@@ -2,7 +2,45 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-if (!is_fuchsia_host && !is_fuchsia) {
+if (is_fuchsia_host || is_fuchsia) {
+  import("//build/dart/dart_library.gni")
+  import("//build/dart/dart_tool.gni")
+
+  dart_library("flutter_frontend_server") {
+    disable_analysis = true
+    package_name = "flutter_frontend_server"
+
+    sources = [
+      "server.dart",
+    ]
+
+    deps = [
+      "//third_party/dart-pkg/pub/args",
+      "//third_party/dart-pkg/pub/path",
+      "//third_party/dart-pkg/pub/usage",
+      "//third_party/dart/pkg/build_integration",
+      "//third_party/dart/pkg/dev_compiler",
+      "//third_party/dart/pkg/front_end",
+      "//third_party/dart/pkg/frontend_server",
+      "//third_party/dart/pkg/kernel",
+      "//third_party/dart/pkg/vm",
+    ]
+  }
+
+  dart_tool("frontend_server_tool") {
+    main_dart = "bin/starter.dart"
+    source_dir = "."
+    disable_analysis = true
+    output_name = "frontend_server"
+
+    sources = []
+
+    deps = [
+      ":flutter_frontend_server",
+    ]
+
+  }
+} else {
   import("//third_party/dart/utils/application_snapshot.gni")
 
   frontend_server_files =