tests changes after collections mapping

compiler/testData/codegen/regressions/kt1397.kt

@@ -1,7 +1,7 @@
 import java.util.ArrayList
 
 class IntArrayList(): ArrayList<Int>() {
-    override fun get(index: Int): Int = super.get(index)!!
+    override fun get(index: Int): Int = super.get(index)
 }
 
 fun box(): String {

compiler/testData/codegen/regressions/kt2395.kt

@@ -2,7 +2,7 @@ import java.util.AbstractList
 
 class MyList(): AbstractList<String>() {
      public fun getModificationCount(): Int = modCount
-     public override fun get(index: Int): String? = ""
+     public override fun get(index: Int): String = ""
      public override fun size(): Int = 0
 }
 

compiler/testData/codegen/regressions/kt950.jet

@@ -1,6 +1,6 @@
 import java.util.*
 
-fun <K, V> Map<K, V>.set(k : K, v : V) = put(k, v)
+fun <K, V> MutableMap<K, V>.set(k : K, v : V) = put(k, v)
 
 fun box() : String {
     val map = HashMap<String,String>()