retrofit try-catch-finally as block-scoped (#4178)

- support optional catch binding

lib/ast.js

@@ -702,28 +702,30 @@ var AST_Try = DEFNODE("Try", "bcatch bfinally", {
             if (!(this.bfinally instanceof AST_Finally)) throw new Error("bfinally must be AST_Finally");
         }
     },
-}, AST_Block);
+}, AST_BlockScope);
 
 var AST_Catch = DEFNODE("Catch", "argname", {
     $documentation: "A `catch` node; only makes sense as part of a `try` statement",
     $propdoc: {
-        argname: "[AST_SymbolCatch] symbol for the exception"
+        argname: "[AST_SymbolCatch?] symbol for the exception, or null if not present",
     },
     walk: function(visitor) {
         var node = this;
         visitor.visit(node, function() {
-            node.argname.walk(visitor);
+            if (node.argname) node.argname.walk(visitor);
             walk_body(node, visitor);
         });
     },
     _validate: function() {
-        if (!(this.argname instanceof AST_SymbolCatch)) throw new Error("argname must be AST_SymbolCatch");
+        if (this.argname != null) {
+            if (!(this.argname instanceof AST_SymbolCatch)) throw new Error("argname must be AST_SymbolCatch");
+        }
     },
 }, AST_BlockScope);
 
 var AST_Finally = DEFNODE("Finally", null, {
     $documentation: "A `finally` node; only makes sense as part of a `try` statement"
-}, AST_Block);
+}, AST_BlockScope);
 
 /* -----[ VAR ]----- */