[fuchsia] Fix build. Build packages/ output. Add Directory::Exists().

runtime/bin/BUILD.gn

 # Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 # for details. All rights reserved. Use of this source code is governed by a
 # BSD-style license that can be found in the LICENSE file.
 
 
 declare_args() {
   dart_io_support = false
 
   # TODO(zra, jamesr): Remove this check once we start building boringssl for
   # the fuchsia port.
@@ skipping 469 matching lines @@
                 "..:dart_product_config",
                 "..:dart_precompiled_runtime_config",]
     deps = [
       ":hello_fuchsia",
       ":gen_resources_cc",
       ":embedded_dart_io",
       ":libdart_builtin",
       "../vm:libdart_platform",
       "..:libdart",
       ":dart_snapshot_cc",
+      "../../pkg:pkg",  # Pull this out to top-level for a real SDK build.
       "//third_party/zlib",
     ]
 
     defines = [
       "NO_OBSERVATORY",
     ]
 
     sources = [
       "main.cc",
       "observatory_assets_empty.cc",
@@ skipping 86 matching lines @@
 
   executable("run_vm_tests_fuchsia") {
     testonly = true
     configs += ["..:dart_config"]
     sources = [
       "run_vm_tests_fuchsia.cc"
     ]
     libs = [ "launchpad" ]
   }
 }  # defined(is_fuchsia) && is_fuchsia