Keep unused function arguments by default

Discarding unused function arguments affects function.length, which can lead
to some hard to debug issues.  This optimization is now done only in "unsafe
mode".

Fix #121

README.md

@@ -279,7 +279,8 @@ when this flag is on:
 - `typeof foo == "undefined"` → `foo === void 0`
 - `void 0` → `undefined` (if there is a variable named "undefined" in
   scope; we do it because the variable name will be mangled, typically
-  reduced to a single character).
+  reduced to a single character)
+- discards unused function arguments (affects `function.length`)
 
 ### Conditional compilation
 

lib/compress.js

@@ -1086,7 +1086,7 @@ merge(Compressor.prototype, {
             var tt = new TreeTransformer(
                 function before(node, descend, in_list) {
                     if (node instanceof AST_Lambda && !(node instanceof AST_Accessor)) {
-                        if (!compressor.option("keep_fargs")) {
+                        if (compressor.option("unsafe") && !compressor.option("keep_fargs")) {
                             for (var a = node.argnames, i = a.length; --i >= 0;) {
                                 var sym = a[i];
                                 if (sym.unreferenced()) {

test/compress/drop-unused.js

@@ -1,5 +1,5 @@
 unused_funarg_1: {
-    options = { unused: true };
+    options = { unused: true, unsafe: true };
     input: {
         function f(a, b, c, d, e) {
             return a + b;
@@ -13,7 +13,7 @@ unused_funarg_1: {
 }
 
 unused_funarg_2: {
-    options = { unused: true };
+    options = { unused: true, unsafe: true };
     input: {
         function f(a, b, c, d, e) {
             return a + c;
@@ -165,7 +165,7 @@ used_var_in_catch: {
 }
 
 keep_fnames: {
-    options = { unused: true, keep_fnames: true };
+    options = { unused: true, keep_fnames: true, unsafe: true };
     input: {
         function foo() {
             return function bar(baz) {};