Provide extension unsafeCast for dynamic.

#KT-15310

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
+}