Implement modular compilation

test/codegen/expect/8invalid-chars.in+file_name.js

 dart_library.library('8invalid-chars.in+file_name', null, /* Imports */[
-  'dart/_runtime'
-], /* Lazy imports */[
-], function(exports, dart) {
+  'dart_sdk'
+], function(exports, dart_sdk) {
   'use strict';
-  let dartx = dart.dartx;
-  function main() {
-  }
-  dart.fn(main);
+  const core = dart_sdk.core;
+  const dart = dart_sdk.dart;
+  const dartx = dart_sdk.dartx;
+  const $8invalid$45chars$46in$43file_name = Object.create(null);
+  $8invalid$45chars$46in$43file_name.main = function() {
+  };
+  dart.fn($8invalid$45chars$46in$43file_name.main);
   // Exports:
-  exports.main = main;
+  exports.$8invalid$45chars$46in$43file_name = $8invalid$45chars$46in$43file_name;
 });