Implement hack to support both remove() and removeAt() in MutableList<Int>

Also add couple of tests about CharSequence.get

compiler/testData/codegen/boxWithJava/collections/charSequence/J.java

@@ -0,0 +1,24 @@
+import java.util.*;
+
+public class J {
+
+    public static class B extends A {
+        public char get(int index) {
+            if (index == 1) return 'a';
+            return super.get(index);
+        }
+    }
+
+    public static String foo() {
+        B b = new B();
+        CharSequence cs = (CharSequence) b;
+
+        if (cs.charAt(0) != 'z') return "fail 1";
+        if (b.get(0) != 'z') return "fail 2";
+
+        if (cs.charAt(1) != 'a') return "fail 3";
+        if (b.get(1) != 'a') return "fail 4";
+
+        return "OK";
+    }
+}