Harden charset-as-string taking function deprecations.

Fix usages in compiler.

libraries/stdlib/test/io/ReadWrite.kt

@@ -15,12 +15,12 @@ fun sample(): Reader = StringReader("Hello\nWorld");
 class ReadWriteTest {
     @test fun testAppendText() {
         val file = File.createTempFile("temp", System.nanoTime().toString())
-        file.writeText("Hello\n", "UTF8")
-        file.appendText("World\n", "UTF8")
+        file.writeText("Hello\n")
+        file.appendText("World\n")
         file.appendText("Again")
 
         assertEquals("Hello\nWorld\nAgain", file.readText())
-        assertEquals(listOf("Hello", "World", "Again"), file.readLines("UTF8"))
+        assertEquals(listOf("Hello", "World", "Again"), file.readLines(Charsets.UTF_8))
         file.deleteOnExit()
     }
 
@@ -145,7 +145,7 @@ class ReadWriteTest {
         val url = URL("http://kotlinlang.org")
         val text = url.readText()
         assertFalse(text.isEmpty())
-        val text2 = url.readText("UTF8")
+        val text2 = url.readText(charset("UTF8"))
         assertFalse(text2.isEmpty())
     }
 }