Fix spidermonkey AST (ESTree) export and import, Array holes

Fixes: #1156 #1161

Also add test to exercise Uglify after spidermonkey export/import of itself.

test/mocha/spidermonkey.js

@@ -0,0 +1,33 @@
+var assert = require("assert");
+var exec = require("child_process").exec;
+
+describe("spidermonkey export/import sanity test", function() {
+    it("should produce a functional build when using --self with spidermonkey", function (done) {
+        this.timeout(20000);
+
+        var uglifyjs = '"' + process.argv[0] + '" bin/uglifyjs';
+        var command = uglifyjs + " --self -cm --wrap SpiderUglify --dump-spidermonkey-ast | " +
+            uglifyjs + " --spidermonkey -cm";
+
+        exec(command, function (err, stdout) {
+            if (err) throw err;
+
+            eval(stdout);
+            assert.strictEqual(typeof SpiderUglify, "object");
+
+            var ast = SpiderUglify.parse("foo([true,,2+3]);");
+            assert.strictEqual(true, ast instanceof SpiderUglify.AST_Node);
+
+            ast.figure_out_scope();
+            ast = SpiderUglify.Compressor({}).compress(ast);
+            assert.strictEqual(true, ast instanceof SpiderUglify.AST_Node);
+
+            var stream = SpiderUglify.OutputStream({});
+            ast.print(stream);
+            var code = stream.toString();
+            assert.strictEqual(code, "foo([!0,,5]);");
+
+            done();
+        });
+    });
+});