Move runtime js files down to lib/runtime/dart

test/codegen/expect/8invalid-chars.in+file_name.js

 dart_library.library('8invalid-chars.in+file_name', null, /* Imports */[
-  "dart_runtime/dart"
+  "dart/_runtime"
 ], /* Lazy imports */[
 ], function(exports, dart) {
   'use strict';
   let dartx = dart.dartx;
   function main() {
   }
   dart.fn(main);
   // Exports:
   exports.main = main;
 });