Turn flutter_tools into a proper host tool. (#9054)

packages/flutter_tools/BUILD.gn

@@ -2,13 +2,15 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-import("//build/dart/dart_package.gni")
+import("//build/dart/dart_tool.gni")
 
-dart_package("flutter_tools") {
+dart_tool("flutter_tools") {
   package_name = "flutter_tools"
 
   source_dir = "lib"
 
+  main_dart = "bin/fuchsia_builder.dart"
+
   deps = [
     "//dart/pkg/analyzer",
     "//dart/third_party/pkg/linter",