Implement modular compilation

test/codegen/expect/collection/src/utils.js

-dart_library.library('collection/src/utils', null, /* Imports */[
-  'dart/_runtime',
-  'dart/core'
-], /* Lazy imports */[
-], function(exports, dart, core) {
-  'use strict';
-  let dartx = dart.dartx;
-  const Pair$ = dart.generic(function(E, F) {
-    class Pair extends core.Object {
-      Pair(first, last) {
-        this.first = first;
-        this.last = last;
-      }
-    }
-    dart.setSignature(Pair, {
-      constructors: () => ({Pair: [Pair$(E, F), [E, F]]})
-    });
-    return Pair;
-  });
-  let Pair = Pair$();
-  function defaultCompare() {
-    return dart.fn((value1, value2) => dart.as(value1, core.Comparable)[dartx.compareTo](value2), core.int, [dart.dynamic, dart.dynamic]);
-  }
-  dart.fn(defaultCompare, () => dart.definiteFunctionType(core.Comparator, []));
-  // Exports:
-  exports.Pair$ = Pair$;
-  exports.Pair = Pair;
-  exports.defaultCompare = defaultCompare;
-});