Dart replacing the node in the abstract syntax tree

Here is the solution for that.

import 'package:analyzer/analyzer.dart';
import 'package:analyzer/dart/ast/token.dart';
import 'package:analyzer/src/dart/ast/token.dart';

String src = """
void main(){
String a ="hey";
String b ="there"; 
print(\$a);
}
""";

AstNode ast;

main() async {

  ast = parseCompilationUnit(src, parseFunctionBodies: true);
  print(ast.toString());
  Visitor v = new Visitor();
  ast.visitChildren(v);
  print(ast.toString());
}    

class Visitor extends SimpleAstVisitor {
  @override
  visitFunctionDeclaration(FunctionDeclaration node) {
    for (var cn in node.childEntities) {
      if (cn.runtimeType.toString() == "FunctionExpressionImpl") {
        Visitor v = new Visitor();
        cn.visitChildren(v);
      }
    }
  }

  @override
  visitBlockFunctionBody(BlockFunctionBody node) {
    Visitor v = new Visitor();
    node.visitChildren(v);
 }

  @override
   visitBlock(Block node) {
     for (var cn in node.childEntities) {
       if (cn.runtimeType.toString() == "ExpressionStatementImpl") {
         Visitor v = new Visitor();
         cn.visitChildren(v);
       }
     }
   }

  @override
  visitMethodInvocation(MethodInvocation node) {
  String result = "print(\$a\$b)";
  Token resultToken =
    new StringToken(TokenType.STRING, result, node.beginToken.offset);
  AstNode replacement = new SimpleStringLiteral(resultToken, result);
  node.parent.accept(new NodeReplacer(node, replacement));
 }
}