Implement modular compilation

test/codegen/expect/map_keys.js

 dart_library.library('map_keys', null, /* Imports */[
-  'dart/_runtime',
-  'dart/core',
-  'dart/math'
-], /* Lazy imports */[
-], function(exports, dart, core, math) {
+  'dart_sdk'
+], function(exports, dart_sdk) {
   'use strict';
-  let dartx = dart.dartx;
-  function main() {
+  const core = dart_sdk.core;
+  const math = dart_sdk.math;
+  const dart = dart_sdk.dart;
+  const dartx = dart_sdk.dartx;
+  const map_keys = Object.create(null);
+  map_keys.main = function() {
     core.print(dart.map({'1': 2, '3': 4, '5': 6}));
     core.print(dart.map([1, 2, 3, 4, 5, 6]));
     core.print(dart.map({'1': 2, [`${dart.notNull(math.Random.new().nextInt(2)) + 2}`]: 4, '5': 6}));
     let x = '3';
     core.print(dart.map(['1', 2, x, 4, '5', 6]));
     core.print(dart.map(['1', 2, null, 4, '5', 6]));
-  }
-  dart.fn(main);
+  };
+  dart.fn(map_keys.main);
   // Exports:
-  exports.main = main;
+  exports.map_keys = map_keys;
 });
+
 //# sourceMappingURL=map_keys.js.map