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/j+k/kt6720_abstractProperty.kt

@@ -11,8 +11,8 @@ public abstract class B implements A {
 
 // FILE: C.kt
 
-<!ABSTRACT_MEMBER_NOT_IMPLEMENTED!>class C<!> : B()
+<!ABSTRACT_CLASS_MEMBER_NOT_IMPLEMENTED!>class C<!> : B()
 
 fun main() {
     C().a
-}
+}