Fix stdlib tests compilation

libraries/stdlib/test/StringJVMTest.kt

@@ -144,7 +144,7 @@ class StringJVMTest {
 
     test fun flatMap() {
         val data = "abcd"
-        val result = data.flatMap { Collections.singletonList(it) }
+        val result = data.flatMap { listOf(it) }
         assertEquals(data.size, result.count())
         assertEquals(data.toCharList(), result)
     }

libraries/stdlib/test/StringTest.kt

@@ -1,6 +1,5 @@
 package test
 
-import java.util.Collections
 import kotlin.test.*
 import org.junit.Test as test