Converter tests:

Change test data files extension to "java"

j2k/tests/testData/ast/assignmentExpression/nullability.java

@@ -0,0 +1,10 @@
+//file
+import java.util.BitSet;
+
+class Foo {
+  void foo(BitSet o) {
+    BitSet o2 = o;
+    int foo = 0;
+    foo = o2.size();
+  }
+}