Fix AssertionError on property generation in multi-file classes

See 4a533168d9 for the original change which
introduced the problem

Note that the added test case _was not failing_ before the change. It's added
because there were no tests on multi-file class behavior in light classes mode
at all. The actual repro for the problem is difficult to make a test from

 #KT-12755 Fixed

compiler/backend/src/org/jetbrains/kotlin/codegen/PropertyCodegen.java

@@ -120,7 +120,6 @@ public class PropertyCodegen {
         assert kind == OwnerKind.PACKAGE || kind == OwnerKind.IMPLEMENTATION || kind == OwnerKind.DEFAULT_IMPLS
                 : "Generating property with a wrong kind (" + kind + "): " + descriptor;
 
-        assert declaration != null : "Declaration is null: " + descriptor + " (context=" + context + ")";
         genBackingFieldAndAnnotations(declaration, descriptor, false);
 
         if (isAccessorNeeded(declaration, descriptor, getter)) {
@@ -131,8 +130,10 @@ public class PropertyCodegen {
         }
     }
 
-    private void genBackingFieldAndAnnotations(@NotNull KtNamedDeclaration declaration, @NotNull PropertyDescriptor descriptor, boolean isParameter) {
-        boolean hasBackingField = hasBackingField(declaration, descriptor);
+    private void genBackingFieldAndAnnotations(
+            @Nullable KtNamedDeclaration declaration, @NotNull PropertyDescriptor descriptor, boolean isParameter
+    ) {
+        boolean hasBackingField = hasBackingField(descriptor);
         boolean hasDelegate = declaration instanceof KtProperty && ((KtProperty) declaration).hasDelegate();
 
         AnnotationSplitter annotationSplitter =
@@ -150,6 +151,7 @@ public class PropertyCodegen {
         if (isBackingFieldOwner) {
             Annotations fieldAnnotations = annotationSplitter.getAnnotationsForTarget(AnnotationUseSiteTarget.FIELD);
             Annotations delegateAnnotations = annotationSplitter.getAnnotationsForTarget(AnnotationUseSiteTarget.PROPERTY_DELEGATE_FIELD);
+            assert declaration != null : "Declaration is null: " + descriptor + " (context=" + context + ")";
             generateBackingField(declaration, descriptor, fieldAnnotations, delegateAnnotations);
             generateSyntheticMethodIfNeeded(descriptor, propertyAnnotations);
         }
@@ -239,7 +241,7 @@ public class PropertyCodegen {
         mv.visitEnd();
     }
 
-    private boolean hasBackingField(@NotNull KtNamedDeclaration p, @NotNull PropertyDescriptor descriptor) {
+    private boolean hasBackingField(@NotNull PropertyDescriptor descriptor) {
         return !isJvmInterface(descriptor.getContainingDeclaration()) &&
                kind != OwnerKind.DEFAULT_IMPLS &&
                !Boolean.FALSE.equals(bindingContext.get(BindingContext.BACKING_FIELD_REQUIRED, descriptor));

compiler/testData/asJava/lightClasses/facades/MultiFile.java

@@ -0,0 +1,3 @@
+public final class MultiFile {
+    public static final int getFoo() { /* compiled code */ }
+}

compiler/testData/asJava/lightClasses/facades/MultiFile.kt

@@ -0,0 +1,8 @@
+// test.MultiFile
+
+@file:JvmMultifileClass
+@file:JvmName("MultiFile")
+
+package test
+
+val foo = 42

compiler/tests/org/jetbrains/kotlin/asJava/CompilerLightClassTestGenerated.java

@@ -88,6 +88,12 @@ public class CompilerLightClassTestGenerated extends AbstractCompilerLightClassT
             doTest(fileName);
         }
 
+        @TestMetadata("MultiFile.kt")
+        public void testMultiFile() throws Exception {
+            String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/asJava/lightClasses/facades/MultiFile.kt");
+            doTest(fileName);
+        }
+
         @TestMetadata("SingleFile.kt")
         public void testSingleFile() throws Exception {
             String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/asJava/lightClasses/facades/SingleFile.kt");

idea/tests/org/jetbrains/kotlin/idea/caches/resolve/IdeCompiledLightClassTestGenerated.java

@@ -76,6 +76,12 @@ public class IdeCompiledLightClassTestGenerated extends AbstractIdeCompiledLight
             KotlinTestUtils.assertAllTestsPresentByMetadata(this.getClass(), new File("compiler/testData/asJava/lightClasses/facades"), Pattern.compile("^([^\\.]+)\\.kt$"), true);
         }
 
+        @TestMetadata("MultiFile.kt")
+        public void testMultiFile() throws Exception {
+            String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/asJava/lightClasses/facades/MultiFile.kt");
+            doTest(fileName);
+        }
+
         @TestMetadata("SingleFile.kt")
         public void testSingleFile() throws Exception {
             String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/asJava/lightClasses/facades/SingleFile.kt");

idea/tests/org/jetbrains/kotlin/idea/caches/resolve/IdeLightClassTestGenerated.java

@@ -61,6 +61,12 @@ public class IdeLightClassTestGenerated extends AbstractIdeLightClassTest {
             doTest(fileName);
         }
 
+        @TestMetadata("MultiFile.kt")
+        public void testMultiFile() throws Exception {
+            String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/asJava/lightClasses/facades/MultiFile.kt");
+            doTest(fileName);
+        }
+
         @TestMetadata("SingleFile.kt")
         public void testSingleFile() throws Exception {
             String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/asJava/lightClasses/facades/SingleFile.kt");