Support for synthetic extensions in codegen

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

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