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

Also add couple of tests about CharSequence.get

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

@@ -0,0 +1,20 @@
+import java.util.*;
+
+public class J {
+
+    private static class MyList extends A {}
+
+    public static String foo() {
+        MyList myList = new MyList();
+        List<Integer> list = (List<Integer>) myList;
+
+        if (!list.remove((Integer) 1)) return "fail 1";
+        if (list.remove((int) 1) != 123) return "fail 2";
+
+        if (!myList.remove((Integer) 1)) return "fail 3";
+        if (myList.remove((int) 1) != 123) return "fail 4";
+
+        if (myList.removeAt(1) != 123) return "fail 5";
+        return "OK";
+    }
+}

compiler/testData/codegen/boxWithJava/collections/removeAtInt/removeAtInt.kt

@@ -0,0 +1,80 @@
+open class A : MutableList<Int> {
+    override val size: Int
+        get() = throw UnsupportedOperationException()
+    override val isEmpty: Boolean
+        get() = throw UnsupportedOperationException()
+
+    override fun contains(o: Int): Boolean {
+        throw UnsupportedOperationException()
+    }
+
+    override fun containsAll(c: Collection<Int>): Boolean {
+        throw UnsupportedOperationException()
+    }
+
+    override fun get(index: Int): Int {
+        throw UnsupportedOperationException()
+    }
+
+    override fun indexOf(o: Any?): Int {
+        throw UnsupportedOperationException()
+    }
+
+    override fun lastIndexOf(o: Any?): Int {
+        throw UnsupportedOperationException()
+    }
+
+    override fun add(e: Int): Boolean {
+        throw UnsupportedOperationException()
+    }
+
+    override fun remove(o: Int) = true
+
+    override fun removeAt(index: Int): Int = 123
+
+    override fun addAll(c: Collection<Int>): Boolean {
+        throw UnsupportedOperationException()
+    }
+
+    override fun addAll(index: Int, c: Collection<Int>): Boolean {
+        throw UnsupportedOperationException()
+    }
+
+    override fun removeAll(c: Collection<Any?>): Boolean {
+        throw UnsupportedOperationException()
+    }
+
+    override fun retainAll(c: Collection<Any?>): Boolean {
+        throw UnsupportedOperationException()
+    }
+
+    override fun clear() {
+        throw UnsupportedOperationException()
+    }
+
+    override fun set(index: Int, element: Int): Int {
+        throw UnsupportedOperationException()
+    }
+
+    override fun add(index: Int, element: Int) {
+        throw UnsupportedOperationException()
+    }
+
+    override fun listIterator(): MutableListIterator<Int> {
+        throw UnsupportedOperationException()
+    }
+
+    override fun listIterator(index: Int): MutableListIterator<Int> {
+        throw UnsupportedOperationException()
+    }
+
+    override fun subList(fromIndex: Int, toIndex: Int): MutableList<Int> {
+        throw UnsupportedOperationException()
+    }
+
+    override fun iterator(): MutableIterator<Int> {
+        throw UnsupportedOperationException()
+    }
+}
+
+fun box() = J.foo()