Implement modular compilation

test/codegen/expect/script.js

 dart_library.library('script', null, /* Imports */[
-  'dart/_runtime',
-  'dart/core'
-], /* Lazy imports */[
-], function(exports, dart, core) {
+  'dart_sdk'
+], function(exports, dart_sdk) {
   'use strict';
-  let dartx = dart.dartx;
-  function main(args) {
+  const core = dart_sdk.core;
+  const dart = dart_sdk.dart;
+  const dartx = dart_sdk.dartx;
+  const script = Object.create(null);
+  script.main = function(args) {
     let name = args[dartx.join](' ');
     if (name == '') name = 'world';
     core.print(`hello ${name}`);
-  }
-  dart.fn(main, dart.void, [core.List$(core.String)]);
+  };
+  dart.fn(script.main, dart.void, [core.List$(core.String)]);
   // Exports:
-  exports.main = main;
+  exports.script = script;
 });