Refactor runtime into libraries, better type reps

lib/runtime/dart/_js_primitives.js

-dart.library('dart/_js_primitives', null, /* Imports */[
+dart_library.library('dart/_js_primitives', null, /* Imports */[
+  "dart_runtime/dart",
   'dart/core'
 ], /* Lazy imports */[
-], function(exports, core) {
+], function(exports, dart, core) {
   'use strict';
+  let dartx = dart.dartx;
   function printString(string) {
     if (typeof dartPrint == "function") {
       dartPrint(string);
       return;
     }
     if (typeof console == "object" && typeof console.log != "undefined") {
       console.log(string);
       return;
     }
     if (typeof window == "object") {
       return;
     }
     if (typeof print == "function") {
       print(string);
       return;
     }
     throw "Unable to print message: " + String(string);
   }
   dart.fn(printString, dart.void, [core.String]);
   // Exports:
   exports.printString = printString;
 });