Don't mix strings with directives in output

* Don't interpret strings with escaped content as directive
 * Don't interpret strings after empty statement as directive
 * Adapt output to prevent strings being represent as directive
 * Introduce UGLIFY_DEBUG to allow internal testing like EXPECT_DIRECTIVE

test/run-tests.js

@@ -1,5 +1,7 @@
 #! /usr/bin/env node
 
+global.UGLIFY_DEBUG = true;
+
 var U = require("../tools/node");
 var path = require("path");
 var fs = require("fs");