Fix output for arrays whose last element is a hole: [1,,]

1529ab96 started to do this (by considering holes to be separate from
"undefined") but it still converted
   [1,,]    (length 2, last element hole, trailing comma)
to
   [1,]     (length 1, trailing comma)

Unfortunately the test suite doesn't really make this clear: the new test here
passes with or without this patch because run-tests.js beautifys the expected
output (in "make_code"), which does the incorrect transformation! If you make
some manual change to arrays.js to make the test fail and see the INPUT and
OUTPUT, then you can see that without this fix, [1,,] -> [1,], and with this fix
it stays [1,,].

lib/output.js

@@ -1007,6 +1007,11 @@ function OutputStream(options) {
             a.forEach(function(exp, i){
                 if (i) output.comma();
                 exp.print(output);
+                // If the final element is a hole, we need to make sure it
+                // doesn't look like a trailing comma, by inserting an actual
+                // trailing comma.
+                if (i === len - 1 && exp instanceof AST_Hole)
+                  output.comma();
             });
             if (len > 0) output.space();
         });

test/compress/arrays.js

@@ -1,10 +1,12 @@
 holes_and_undefined: {
     input: {
+        w = [1,,];
         x = [1, 2, undefined];
         y = [1, , 2, ];
         z = [1, undefined, 3];
     }
     expect: {
+        w=[1,,];
         x=[1,2,void 0];
         y=[1,,2];
         z=[1,void 0,3];