Is there a way to check if the script is running in the dart vm or dart2js?

There is no official way, as far as I know. The intent is that for all practical purposes, you shouldn't have to know if you are running native or compiled to JavaScript.

That said, there are few hacks you can use. The easiest one is probably to exploit the fact that Dart has two numeric types, int and double, while JavaScript has only one, which is equivalent to Dart's double, and dart2js doesn't have a special implementation of int just yet. Therefore, identical(1, 1.0) is false in Dart, and the VM implements that correctly, but when compiled to JS, it is true.

Note that you should think pretty hard before using a hack like this. In most cases, you don't have to do that, just write Dart and don't try to recognize if you are running JS or not. Also, noone can guarantee that it will work forever.

Based on a code fragment found in path library (Dart v0.7.2) :

import 'dart:mirrors';/// Value indicating that the VM is Dart on server-side.const int DART_SERVER=1;/// Value indicating that the VM is Dart on client-side.const int DART_CLIENT=2;/// Value indicating that the VM is JavaScript on client-side (e.g. dart2js).const int JS_CLIENT=3;/// Returns the type of the current virtual machine.int vmType() {  Map<Uri, LibraryMirror> libraries=currentMirrorSystem().libraries;  if(libraries[Uri.parse('dart:io')]!=null) return DART_SERVER;  if(libraries[Uri.parse('dart:html')]!=null) return DART_CLIENT;  return JS_CLIENT;}/// Application entry point.void main() {  print(vmType());}

Another very hacky way to for a script to tell if is running in the server side VM vs the client side is to use the Options class.

The following app (file called myapp.dart) prints different output when run on the command line VM or in the browser:

void main() {  var options = new Options();  print(options.arguments);  print(options.executable);  print(options.script);}

Command line output:

[]C:\work\dart\editor\dart\dart-sdk\bin\dart.exemyapp.dart

In-browser output:

[]     <-- empty string     <-- empty string

The in-browser output is the same, though, whether it's running in a Dart browser VM or as JS.