added visitor for stubstituting mixin (not working)

src/tools/debugger/visitors/UpdateMixinIdVisitor.java

@@ -0,0 +1,24 @@
+package tools.debugger.visitors;
+
+import com.oracle.truffle.api.instrumentation.InstrumentableNode;
+import com.oracle.truffle.api.nodes.Node;
+import com.oracle.truffle.api.nodes.NodeVisitor;
+import com.oracle.truffle.api.nodes.RootNode;
+import som.compiler.MixinBuilder;
+
+public class UpdateMixinIdVisitor implements NodeVisitor {
+    private MixinBuilder.MixinDefinitionId mixinDefinitionId;
+
+    public UpdateMixinIdVisitor(MixinBuilder.MixinDefinitionId mixinDefinitionId) {
+        this.mixinDefinitionId = mixinDefinitionId;
+    }
+
+    @Override
+    public boolean visit(Node node) {
+        return true;
+    }
+
+    public MixinBuilder.MixinDefinitionId getMixinDefinitionId() {
+        return mixinDefinitionId;
+    }
+}