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

mojo/public/mojo_sdk.gni

Index: mojo/public/mojo_sdk.gni
diff --git a/mojo/public/mojo_sdk.gni b/mojo/public/mojo_sdk.gni
index dbed84c36e15257e37d6c3a20289383a4829f7cc..161a229d288d37504c824e89b8f748f4e4a466a5 100644
--- a/mojo/public/mojo_sdk.gni
+++ b/mojo/public/mojo_sdk.gni
@@ -123,7 +123,9 @@ template("mojo_sdk_source_set") {
           # the Mojo SDK should be on targets within the same file or on a
           # whitelisted set of external dependencies.
           dep_dir = get_path_info(dep, "dir")
-          assert(dep_dir == "." || dep == "//testing/gtest")
+          assert(dep_dir == "." || dep == "//testing/gtest" ||
+                 (dep == "//dart/runtime:libdart" &&
+                  target_name == "mojo_internal_impl"))
         }
         deps += [ dep ]
       }