enhance keep_fargs (#3409)

lib/ast.js

@@ -376,7 +376,7 @@ var AST_Toplevel = DEFNODE("Toplevel", "globals", {
     }
 }, AST_Scope);
 
-var AST_Lambda = DEFNODE("Lambda", "name argnames uses_arguments", {
+var AST_Lambda = DEFNODE("Lambda", "name argnames uses_arguments length_read", {
     $documentation: "Base class for functions",
     $propdoc: {
         name: "[AST_SymbolDeclaration?] the name of this function",
@@ -614,6 +614,18 @@ var AST_PropAccess = DEFNODE("PropAccess", "expression property", {
     $propdoc: {
         expression: "[AST_Node] the “container” expression",
         property: "[AST_Node|string] the property to access.  For AST_Dot this is always a plain string, while for AST_Sub it's an arbitrary AST_Node"
+    },
+    getProperty: function() {
+        var p = this.property;
+        if (p instanceof AST_Constant) {
+            return p.getValue();
+        }
+        if (p instanceof AST_UnaryPrefix
+            && p.operator == "void"
+            && p.expression instanceof AST_Constant) {
+            return;
+        }
+        return p;
     }
 });