Refactor runtime into libraries, better type reps

lib/runtime/dart/_js_helper.js

-dart.library('dart/_js_helper', null, /* Imports */[
+dart_library.library('dart/_js_helper', null, /* Imports */[
+  "dart_runtime/dart",
   'dart/core',
   'dart/collection',
   'dart/_interceptors',
   'dart/_foreign_helper'
 ], /* Lazy imports */[
-], function(exports, core, collection, _interceptors, _foreign_helper) {
+], function(exports, dart, core, collection, _interceptors, _foreign_helper) {
   'use strict';
+  let dartx = dart.dartx;
   class NoThrows extends core.Object {
     NoThrows() {
     }
   }
   dart.setSignature(NoThrows, {
     constructors: () => ({NoThrows: [NoThrows, []]})
   });
   class NoInline extends core.Object {
     NoInline() {
     }
@@ skipping 1174 matching lines @@
   exports.Returns = Returns;
   exports.JSName = JSName;
   exports.JavaScriptIndexingBehavior = JavaScriptIndexingBehavior;
   exports.TypeErrorImplementation = TypeErrorImplementation;
   exports.CastErrorImplementation = CastErrorImplementation;
   exports.FallThroughErrorImplementation = FallThroughErrorImplementation;
   exports.RuntimeError = RuntimeError;
   exports.random64 = random64;
   exports.jsonEncodeNative = jsonEncodeNative;
 });