added option for side-effect-free statements, fix test

lib/compress.js

@@ -64,6 +64,7 @@ function Compressor(options, false_by_default) {
         if_return     : !false_by_default,
         join_vars     : !false_by_default,
         cascade       : !false_by_default,
+        side_effects  : !false_by_default,
 
         warnings      : true,
         global_defs   : {}
@@ -998,9 +999,11 @@ merge(Compressor.prototype, {
     });
 
     OPT(AST_SimpleStatement, function(self, compressor){
-        if (!self.body.has_side_effects()) {
-            compressor.warn("Dropping side-effect-free statement [{file}:{line},{col}]", self.start);
-            return make_node(AST_EmptyStatement, self);
+        if (compressor.option("side_effects")) {
+            if (!self.body.has_side_effects()) {
+                compressor.warn("Dropping side-effect-free statement [{file}:{line},{col}]", self.start);
+                return make_node(AST_EmptyStatement, self);
+            }
         }
         return self;
     });
@@ -1153,7 +1156,7 @@ merge(Compressor.prototype, {
                 operator: "&&",
                 left: self.condition,
                 right: self.body.condition
-            });
+            }).transform(compressor);
             self.body = self.body.body;
         }
         if (aborts(self.body)) {