Enable --screw-ie8 by default.

catch identifier is mangled correctly for ES5 standards-compliant JS engines by default.

Unconditionally use the ie8 if/do-while workaround whether or not --screw-ie8 is enabled.

To support non-standard ie8 javascript use: uglifyjs --support-ie8

test/compress/properties.js

@@ -12,7 +12,8 @@ keep_properties: {
 
 dot_properties: {
     options = {
-        properties: true
+        properties: true,
+        screw_ie8: false
     };
     input: {
         a["foo"] = "bar";