Don't lookup built-in classes by invalid names

#KT-5869 Fixed

compiler/builtins-serializer/src/BuiltInsSerializer.kt

@@ -164,6 +164,6 @@ public class BuiltInsSerializer(val out: PrintStream?) {
     }
 
     fun getFileName(classDescriptor: ClassDescriptor): String {
-        return BuiltInsSerializationUtil.getClassMetadataPath(DeserializedResolverUtils.getClassId(classDescriptor))
+        return BuiltInsSerializationUtil.getClassMetadataPath(DeserializedResolverUtils.getClassId(classDescriptor))!!
     }
 }

compiler/testData/codegen/box/objects/useAnonymousObjectAsIterator.kt

@@ -0,0 +1,18 @@
+// KT-5869
+
+fun <T> Iterator<T>.iterator(): Iterator<T> = this
+
+fun box(): String {
+    val iterator = object : Iterator<Int> {
+        var i = 0
+        override fun next() = i++
+        override fun hasNext() = i < 5
+    }
+
+    var result = ""
+    for (i in iterator) {
+        result += i
+    }
+
+    return if (result == "01234") "OK" else "Fail $result"
+}

compiler/tests/org/jetbrains/jet/codegen/generated/BlackBoxCodegenTestGenerated.java

@@ -4971,6 +4971,12 @@ public class BlackBoxCodegenTestGenerated extends AbstractBlackBoxCodegenTest {
             doTest(fileName);
         }
         
+        @TestMetadata("useAnonymousObjectAsIterator.kt")
+        public void testUseAnonymousObjectAsIterator() throws Exception {
+            String fileName = JetTestUtils.navigationMetadata("compiler/testData/codegen/box/objects/useAnonymousObjectAsIterator.kt");
+            doTest(fileName);
+        }
+        
     }
     
     @TestMetadata("compiler/testData/codegen/box/operatorConventions")