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/codegen/box/builtinsProperties/bridges.kt

@@ -9,6 +9,8 @@ class B0 : Collection<String>, A0 {
     override fun contains(o: String) = throw UnsupportedOperationException()
     override fun iterator() = throw UnsupportedOperationException()
     override fun containsAll(c: Collection<String>) = throw UnsupportedOperationException()
+    override val size: Int
+        get() = super.size
 }
 
 open class A1 {
@@ -45,6 +47,8 @@ class B4 : Collection<String>, I4<Int> {
     override fun contains(o: String) = throw UnsupportedOperationException()
     override fun iterator() = throw UnsupportedOperationException()
     override fun containsAll(c: Collection<String>) = throw UnsupportedOperationException()
+    override val size: Int
+        get() = super.size
 }
 
 interface I5 : Collection<String> {