Implement modular compilation

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
deleted file mode 100644
index 96dc101858017ea02b54a42c77798a19cba02868..0000000000000000000000000000000000000000
--- a/test/codegen/expect/dir/html_input_a.js
+++ /dev/null
@@ -1,18 +0,0 @@
-dart_library.library('dir/html_input_a', null, /* Imports */[
-  'dart/_runtime',
-  'dart/core',
-  'dir/html_input_b',
-  'dir/html_input_c',
-  'dir/html_input_d'
-], /* Lazy imports */[
-], 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))}`);
-  }
-  dart.fn(main);
-  // Exports:
-  exports.main = main;
-});