Move implementation of dart:mojo.internal to mojo/public/platform/dart

mojo/public/platform/dart/BUILD.gn

+# Copyright 2015 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+import("../../mojo_sdk.gni")
+
+mojo_sdk_source_set("mojo_internal_impl") {
+  sources = [
+    "mojo_natives.cc",
+    "mojo_natives.h",
+  ]
+
+  deps = [
+    "//dart/runtime:libdart",
+  ]
+
+  mojo_sdk_deps = [
+    "mojo/public/c/system:system",
+    "mojo/public/cpp/environment:environment",
+    "mojo/public/cpp/system:system",
+  ]
+}