[JS IR] Fix JS code diagnostics tests according to new KLIB checks

^KT-62425

compiler/testData/diagnostics/testsWithJsStdLibAndBackendCompilation/jsCode/badAssignment.kt

@@ -1,4 +1,9 @@
+// FIR_DIFFERENCE
+// The diagnostic cannot be implemented with the FIR frontend checker because it requires constant evaluation over FIR.
+// The diagnostic is implemented as a klib check over IR.
+
+// ERROR_POLICY: SEMANTIC
 // !DIAGNOSTICS: -UNUSED_PARAMETER
 fun Int.foo(x: Int) {
     js("this = x<!JSCODE_ERROR!><!>;")
-}
+}