Interesting Macro error: if you define Example here in the same file as Main.hx then you get the error "Cannot access static field [field] from a class instance", but it's lying. Extract Example.hx to a different file and it compiles (and outputs "hello world", as you would expect).

Main.hx

package;

#if macro
import haxe.macro.Expr;
import haxe.macro.Expr.ExprOf;
import haxe.macro.Context;
import haxe.macro.Type;
#end

class Main
{
    public static function main()
        new Main();

    public function new()
    {
        var instance = new Example();
        instance.write("hello world");
        trace(instance.read());
    }

}

class Example
{
    var map:Map<Int, String>;

    public function new()
    {
        map = new Map<Int, String>();
    }

    macro public function write(self:ExprOf<Example>, value:Expr):Expr
    {
        return macro (untyped $self.hiddenWrite)(1, $value);
    }

      function hiddenWrite(id:Int, value:String)
      {
          map.set(id, value);
      }

    public function read():String
    {
        return map.get(1);
    }

}

// openfl test flash.nmml flash
// src/Main.hx:20: characters 8-22 : Cannot access static field write from a class instance

FYI from Matthew Spencer (@m22spencer on Twitter) - "Separating classes fixes since you are no longer compiling cls Main in macro context"