Java 8 rules for method overrides:

- base class method wins against a (default) interface method,
so an abstract base class method should always be implemented
in a derived class;

- interface methods clash regardless of abstract/default
with possibly undefined behavior at run-time,
so a class or interface should always define its own method
for methods inherited from multiple interfaces and not from base class;

- meaningful diagnostics for class inheriting conflicting JVM signatures.
Since no override will happen under Java 8 rules,
ACCIDENTAL_OVERRIDE is misleading for this case;

- update testData.

compiler/testData/diagnostics/tests/modifiers/const/applicability.kt

@@ -22,7 +22,7 @@ abstract class C {
     }
 }
 
-<!ABSTRACT_MEMBER_NOT_IMPLEMENTED!>object D<!> : C() {
+<!ABSTRACT_CLASS_MEMBER_NOT_IMPLEMENTED!>object D<!> : C() {
     <!INCOMPATIBLE_MODIFIERS!>override<!> <!INCOMPATIBLE_MODIFIERS!>const<!> val x: Int = 9
 
     const val inObject = 10