Support for synthetic extensions in codegen

compiler/testData/codegen/boxAgainstJava/syntheticExtensions/plusPlus.java

@@ -0,0 +1,11 @@
+class JavaClass {
+    private int myX = 0;
+
+    public int getX() {
+        return myX;
+    }
+
+    public void setX(int x) {
+        myX = x;
+    }
+}