Refactor runtime into libraries, better type reps

test/codegen/expect/dir/html_input_a.js

Index: test/codegen/expect/dir/html_input_a.js
diff --git a/test/codegen/expect/dir/html_input_a.js b/test/codegen/expect/dir/html_input_a.js
index ad3c813f99313c072e09eb36584d9d719cf2952b..9c21632a1da7c4ef65dfff88a39e3e0979bff41b 100644
--- a/test/codegen/expect/dir/html_input_a.js
+++ b/test/codegen/expect/dir/html_input_a.js
@@ -1,11 +1,13 @@
-dart.library('dir/html_input_a', null, /* Imports */[
+dart_library.library('dir/html_input_a', null, /* Imports */[
+  "dart_runtime/dart",
   'dart/core',
   'dir/html_input_b',
   'dir/html_input_c',
   'dir/html_input_d'
 ], /* Lazy imports */[
-], function(exports, core, html_input_b, html_input_c, html_input_d) {
+], function(exports, dart, core, html_input_b, html_input_c, html_input_d) {
   'use strict';
+  let dartx = dart.dartx;
   function main() {
     core.print(`fib(${html_input_b.x} + ${html_input_c.y}) = `);
     core.print(`  ... ${html_input_d.fib(dart.notNull(html_input_b.x) + dart.notNull(html_input_c.y))}`);