implement --expression (#5607)

lib/compress.js

@@ -343,6 +343,35 @@ Compressor.prototype.compress = function(node) {
         });
         self.transform(tt);
     });
+    AST_Toplevel.DEFMETHOD("unwrap_expression", function() {
+        var self = this;
+        switch (self.body.length) {
+          case 0:
+            return make_node(AST_UnaryPrefix, self, {
+                operator: "void",
+                expression: make_node(AST_Number, self, { value: 0 }),
+            });
+          case 1:
+            var stat = self.body[0];
+            if (stat instanceof AST_Directive) return make_node(AST_String, self, stat);
+            if (stat instanceof AST_SimpleStatement) return stat.body;
+          default:
+            return make_node(AST_Call, self, {
+                expression: make_node(AST_Function, self, {
+                    argnames: [],
+                    body: self.body,
+                }).init_vars(self),
+                args: [],
+            });
+        }
+        return self;
+    });
+    AST_Node.DEFMETHOD("wrap_expression", function() {
+        var self = this;
+        if (!is_statement(self)) self = make_node(AST_SimpleStatement, self, { body: self });
+        if (!(self instanceof AST_Toplevel)) self = make_node(AST_Toplevel, self, { body: [ self ] });
+        return self;
+    });
 
     function read_property(obj, node) {
         var key = node.get_property();