[PowerAssert] Reproduce KT-66208

plugins/power-assert/testData/codegen/cast/InstanceNotEquals.kt

@@ -1,3 +1,21 @@
 fun box() = expectThrowableMessage {
     assert("Hello, world!" !is String)
+} + "\n\n" + expectThrowableMessage {
+    // Test that we don't just search for `!is` in the expression.
+    assert(" !is " !is String)
+} + "\n\n" + expectThrowableMessage {
+    // Test multiline case
+    assert(
+        " !is "
+
+                !is
+
+                String
+    )
+} + "\n\n" + expectThrowableMessage {
+    // Test that we don't assume whitespaces around the operator
+    assert("Hello, world!"/*!is*/!is/*!is*/String)
+} + "\n\n" + expectThrowableMessage {
+    // Test nested `!is`
+    assert(("Hello, world!" !is String) !is Boolean)
 }