Implement modular compilation

lib/src/js_ast/js_types.dart

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of js_ast;
 
 final _any = new AnyTypeRef._();
 final _unknown = new UnknownTypeRef._();
 final _null = new NullTypeRef();
 
@@ skipping 178 matching lines @@
   accept(NodeVisitor visitor) => visitor.visitFunctionTypeRef(this);
   void visitChildren(NodeVisitor visitor) {
     returnType.accept(visitor);
     paramTypes.forEach((n, t) {
       n.accept(visitor);
       t.accept(visitor);
     });
   }
   _clone() => new FunctionTypeRef(returnType, paramTypes);
 }