Simplify method contract and add assertion

We don't generate stubs for final built-ins anyway
(as well as special bridges)

compiler/backend/src/org/jetbrains/kotlin/codegen/builtinSpecialBridges.kt

@@ -89,8 +89,8 @@ object BuiltinSpecialBridgesUtil {
         val bridges: MutableSet<BridgeForBuiltinSpecial<Signature>> = mutableSetOf()
 
         val superImplementationDescriptor =
-                if (specialBridge != null)
-                    findSuperImplementationForStubDelegation(function, fake, isBodyOwner, signatureByDescriptor)
+                if (specialBridge != null && fake && !functionHandle.isAbstract)
+                    findSuperImplementationForStubDelegation(function, isBodyOwner, signatureByDescriptor)
                 else
                     null
 
@@ -140,17 +140,21 @@ object BuiltinSpecialBridgesUtil {
  */
 private fun <Signature> findSuperImplementationForStubDelegation(
         function: FunctionDescriptor,
-        fake: Boolean,
         isBodyOwner: (DeclarationDescriptor) -> Boolean,
         signatureByDescriptor: (FunctionDescriptor) -> Signature
 ): FunctionDescriptor? {
-    if (function.modality != Modality.OPEN || !fake) return null
     val implementation = findConcreteSuperDeclaration(DescriptorBasedFunctionHandle(function, isBodyOwner)).descriptor
+
+    // Implementation from interface will be generated by common mechanism
     if (DescriptorUtils.isInterface(implementation.containingDeclaration)) return null
 
     // Implementation in super-class already has proper signature
     if (signatureByDescriptor(function) == signatureByDescriptor(implementation)) return null
 
+    assert(function.modality == Modality.OPEN) {
+        "Should generate stubs only for non-abstract built-ins, but ${function.name} is ${function.modality}"
+    }
+
     return implementation
 }