Implement modular compilation

test/js/builder_test.dart

-import 'package:dev_compiler/src/js/js_ast.dart';
+import 'package:dev_compiler/src/js_ast/js_ast.dart';
 import 'package:test/test.dart';
 
 final _prenumberedPlaceholders = new RegExp(r'#\d+');
 
 _parser(String src) =>
     new MiniJsParser(src.replaceAll(_prenumberedPlaceholders, '#'));
 
 _check(Node node, String expected) =>
     expect(node.toString(), 'js_ast `$expected`');
 
@@ skipping 24 matching lines @@
           '}');
     });
     test('parses statements', () {
       _checkStatement('for (let i = 0; i < 10; i++) {\n}\n');
       _checkStatement('for (let i = 0, j = 1; i < 10; i++) {\n}\n');
       _checkStatement('var [x, y = []] = list;\n');
       _checkStatement('var {x, y = {x: y}} = obj;\n');
     });
   });
 }