Implement modular compilation

test/codegen/destructuring.dart

+// compile options: --destructure-named-params
 import 'dart-ext:foo';
 import 'package:js/src/varargs.dart';
 
 f(int a, b, [c = 1]) {
   f(a, b, c);
 }
 external f_ext(int a, b, [c = 1]);
 f_nat(int a, b, [c = 1]) native "f_nat";
 f_sync(int a, b, [c = 1]) sync* {}
 f_async(int a, b, [c = 1]) async* {}
@@ skipping 20 matching lines @@
 invalid_names2([int let, function = 1, arguments]) {
   f(let, function, arguments);
 }
 invalid_names3({int let, function, arguments : 2}) {
   f(let, function, arguments);
 }
 
 names_clashing_with_object_props({int constructor, valueOf, hasOwnProperty : 2}) {
   f(constructor, valueOf, hasOwnProperty);
 }