diff -r 36961c9a009f -r e499b0dddd12 rt/vm/src/main/java/org/apidesign/vm4brwsr/Main.java
--- a/rt/vm/src/main/java/org/apidesign/vm4brwsr/Main.java	Mon May 13 18:54:50 2013 +0200
+++ b/rt/vm/src/main/java/org/apidesign/vm4brwsr/Main.java	Fri May 24 18:04:55 2013 +0200
@@ -97,7 +97,14 @@
                 collectClasses(classes, mainClassLoader, args[i]);
             }
         }
-        try (Writer w = new BufferedWriter(new FileWriter(generateTo))) {
+        
+        File gt = new File(generateTo);
+        if (Boolean.getBoolean("skip.if.exists") && gt.isFile()) {
+            System.err.println("Skipping as " + gt + " exists.");
+            System.exit(0);
+        }
+        
+        try (Writer w = new BufferedWriter(new FileWriter(gt))) {
             Bck2Brwsr.newCompiler().
                 extension(createExtension).
                 obfuscation(obfLevel).