Provide extension unsafeCast for dynamic.

#KT-15310

js/js.libraries/src/core/dynamic.kt

@@ -23,6 +23,13 @@ public inline fun Any?.asDynamic(): dynamic = this
  */
 public inline fun <T> Any?.unsafeCast(): @kotlin.internal.NoInfer T = this.asDynamic()
 
+/**
+ * Reinterprets this `dynamic` value as a value of the specified type [T] without any actual type checking.
+ */
+@kotlin.internal.DynamicExtension
+@JsName("unsafeCastDynamic")
+public inline fun <T> dynamic.unsafeCast(): @kotlin.internal.NoInfer T = this
+
 // TODO add the support ES6 iterators
 @kotlin.internal.DynamicExtension
 public operator fun dynamic.iterator(): Iterator<dynamic> {

js/js.tests/test/org/jetbrains/kotlin/js/test/semantics/BoxJsTestGenerated.java

@@ -4732,6 +4732,12 @@ public class BoxJsTestGenerated extends AbstractBoxJsTest {
             String fileName = KotlinTestUtils.navigationMetadata("js/js.translator/testData/box/inlineStdlib/thisInExtension.kt");
             doTest(fileName);
         }
+
+        @TestMetadata("unsafeCast.kt")
+        public void testUnsafeCast() throws Exception {
+            String fileName = KotlinTestUtils.navigationMetadata("js/js.translator/testData/box/inlineStdlib/unsafeCast.kt");
+            doTest(fileName);
+        }
     }
 
     @TestMetadata("js/js.translator/testData/box/java")

js/js.translator/testData/box/inlineStdlib/unsafeCast.kt

@@ -0,0 +1,13 @@
+package foo
+
+// CHECK_NOT_CALLED_IN_SCOPE: scope=box function=isType
+// CHECK_NOT_CALLED_IN_SCOPE: scope=box function=throwCCE
+
+fun box(): String {
+    // dynamic unsafeCast
+    val result = js("\"OK\"").unsafeCast<String>()
+    // Any unsafeCast
+    val intOk = result.unsafeCast<Int>()
+
+    return result
+}