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

lib/scope.js

@@ -88,7 +88,7 @@ SymbolDef.prototype = {
 
 AST_Toplevel.DEFMETHOD("figure_out_scope", function(options){
     options = defaults(options, {
-        screw_ie8: false,
+        screw_ie8: true,
         cache: null
     });
 
@@ -377,7 +377,7 @@ AST_Toplevel.DEFMETHOD("_default_mangler_options", function(options){
         eval        : false,
         sort        : false, // Ignored. Flag retained for backwards compatibility.
         toplevel    : false,
-        screw_ie8   : false,
+        screw_ie8   : true,
         keep_fnames : false
     });
 });