Revert "remove support for const (#1910)"

This reverts commit c391576d52.

test/mocha/cli.js

@@ -379,6 +379,21 @@ describe("bin/uglifyjs", function () {
            done();
        });
     });
+    it("Should throw syntax error (const a)", function(done) {
+       var command = uglifyjscmd + ' test/input/invalid/const.js';
+
+       exec(command, function (err, stdout, stderr) {
+           assert.ok(err);
+           assert.strictEqual(stdout, "");
+           assert.strictEqual(stderr.split(/\n/).slice(0, 4).join("\n"), [
+               "Parse error at test/input/invalid/const.js:7,11",
+               "    const a;",
+               "           ^",
+               "ERROR: Missing initializer in const declaration"
+           ].join("\n"));
+           done();
+       });
+    });
     it("Should throw syntax error (delete x)", function(done) {
        var command = uglifyjscmd + ' test/input/invalid/delete.js';