Fix output for arrays containing undefined values.

1b6bcca7 was a first attempt at this. That commit made Uglify stop replacing
holes with undefined, but instead it started replacing undefined with
holes. This is slightly problematic, because there is a difference between a
hole and an undefined value. More problematically, it changed [1,undefined] to
[1,] which generally doesn't even parse as a hole (just as a trailing comma), so
it didn't even preserve the length of the array!

Instead, parse holes as their own special AST node which prints invisibly.

lib/output.js

@@ -984,8 +984,7 @@ function OutputStream(options) {
             if (len > 0) output.space();
             a.forEach(function(exp, i){
                 if (i) output.comma();
-                if (!(exp instanceof AST_Undefined))
-                    exp.print(output);
+                exp.print(output);
             });
             if (len > 0) output.space();
         });
@@ -1036,6 +1035,9 @@ function OutputStream(options) {
     DEFPRINT(AST_Undefined, function(self, output){
         output.print("void 0");
     });
+    DEFPRINT(AST_Hole, function(self, output){
+        output.print("");
+    });
     DEFPRINT(AST_Infinity, function(self, output){
         output.print("1/0");
     });