rewrite the constructor for ast

[yimi0]

We can add custom properties to SELF_PROPS to ensure that the properties can be cloned.

[rvanvelzen]

What concrete problem does this solve?

[yimi0]

No problem, just to improve the extensibility. is useful when calling node.clone(true).

[alexlamsl]

Is there any examples that illustrates said improvement, e.g. some mocha tests?

[yimi0]

UglifyJS.AST_Node.SELF_PROPS.push("skip");
UglifyJS.AST_Lambda.SELF_PROPS.push("id", "level");

let id = 0;
ast.walk(new UglifyJS.TreeWalker(node => {
    if (node instanceof UglifyJS.AST_Lambda) {
        node.id = id++;
        node.level = getScopeLevel(node);
        ...
    }
    ...
    node.skip = canSkip(node);
});

...

let oldAst = ast, newAst = ast.clone(true);

newAst.walk(new UglifyJS.TreeWalker(node => {
    if (node.skip) return;
    if (node instanceof UglifyJS.AST_Lambda) {
        switch (node.level) {
             case 0:
                 // toplevel
                 ...
                 break;
             case 1:
             ...
             default:
                 ...
        }
    }
    ...
});