Refactor runtime into libraries, better type reps

test/codegen/expect/sunflower/circle.js

-dart.library('circle', null, /* Imports */[
+loader.library('circle', null, /* Imports */[
+  "dart/dart_runtime",
+  "dart/dartx",
   'dart/core'
 ], /* Lazy imports */[
-], function(exports, core) {
+], function(exports, dart, dartx, core) {
   'use strict';
   class Circle extends core.Object {
     Circle(x, y, radius) {
       this.x = x;
       this.y = y;
       this.radius = radius;
     }
   }
   dart.setSignature(Circle, {
     constructors: () => ({Circle: [Circle, [core.num, core.num, core.num]]})
   });
   // Exports:
   exports.Circle = Circle;
 });