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

tools/exports.js

@@ -17,3 +17,7 @@ exports["string_template"] = string_template;
 exports["tokenizer"] = tokenizer;
 exports["is_identifier"] = is_identifier;
 exports["SymbolDef"] = SymbolDef;
+
+if (DEBUG) {
+    exports["EXPECT_DIRECTIVE"] = EXPECT_DIRECTIVE;
+}