Throw IOOB on invalid index in JS StringBuilder.get to adhere contract

libraries/stdlib/test/text/StringBuilderTest.kt

@@ -75,4 +75,16 @@ class StringBuilderTest {
         assertTrue(sb.isEmpty())
         assertEquals("test", s)
     }
+
+    @Test fun get() {
+        val sb = StringBuilder()
+        sb.append("test")
+        assertEquals('t', sb[0])
+        assertEquals('e', sb[1])
+        assertEquals('s', sb[2])
+        assertEquals('t', sb[3])
+
+        assertFailsWith<IndexOutOfBoundsException> { assertEquals('t', sb[-1]) }
+        assertFailsWith<IndexOutOfBoundsException> { assertEquals('t', sb[4]) }
+    }
 }