Implement modular compilation

lib/runtime/dart/_js_primitives.js

-dart_library.library('dart/_js_primitives', null, /* Imports */[
-  'dart/_runtime',
-  'dart/core'
-], /* Lazy imports */[
-], function(exports, dart, core) {
-  'use strict';
-  let dartx = dart.dartx;
-  function printString(string) {
-    if (typeof dartPrint == "function") {
-      dartPrint(string);
-      return;
-    }
-    if (typeof console == "object" && typeof console.log != "undefined") {
-      console.log(string);
-      return;
-    }
-    if (typeof window == "object") {
-      return;
-    }
-    if (typeof print == "function") {
-      print(string);
-      return;
-    }
-    throw "Unable to print message: " + String(string);
-  }
-  dart.fn(printString, dart.void, [core.String]);
-  // Exports:
-  exports.printString = printString;
-});