fix class expression statements (#2051)

- class expression statements require parentheses
- allow unused class expression statements to be dropped

fixes #1782
closes #1784

lib/compress.js

@@ -2610,6 +2610,7 @@ merge(Compressor.prototype, {
         def(AST_Accessor, return_null);
         def(AST_Function, return_null);
         def(AST_Arrow, return_null);
+        def(AST_ClassExpression, return_null);
         def(AST_Binary, function(compressor, first_in_statement){
             var right = this.right.drop_side_effect_free(compressor);
             if (!right) return this.left.drop_side_effect_free(compressor, first_in_statement);