Refactor runtime into libraries, better type reps

lib/runtime/dart/isolate.js

-dart.library('dart/isolate', null, /* Imports */[
+loader.library('dart/isolate', null, /* Imports */[
+  "dart/dart_runtime",
+  "dart/dartx",
   'dart/core',
   'dart/async'
 ], /* Lazy imports */[
   'dart/_isolate_helper'
-], function(exports, core, async, _isolate_helper) {
+], function(exports, dart, dartx, core, async, _isolate_helper) {
   'use strict';
   class Capability extends core.Object {
     static new() {
       return new _isolate_helper.CapabilityImpl();
     }
   }
   dart.setSignature(Capability, {
     constructors: () => ({new: [Capability, []]})
   });
   class IsolateSpawnException extends core.Object {
@@ skipping 237 matching lines @@
   });
   // Exports:
   exports.Capability = Capability;
   exports.IsolateSpawnException = IsolateSpawnException;
   exports.Isolate = Isolate;
   exports.SendPort = SendPort;
   exports.ReceivePort = ReceivePort;
   exports.RawReceivePort = RawReceivePort;
   exports.RemoteError = RemoteError;
 });