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

libraries/stdlib/js/src/kotlin/text.kt

@@ -24,7 +24,8 @@ public actual class StringBuilder(content: String = "") : Appendable, CharSequen
     actual override val length: Int
         get() = string.asDynamic().length
 
-    actual override fun get(index: Int): Char = string[index]
+    actual override fun get(index: Int): Char =
+        string.getOrElse(index) { throw IndexOutOfBoundsException("index: $index, length: $length}") }
 
     actual override fun subSequence(startIndex: Int, endIndex: Int): CharSequence = string.substring(startIndex, endIndex)
 

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]) }
+    }
 }