KT-25732 Perform keywords completion earlier to complete them before freeze

- If you complete keywords too late, the completion list may become frozen, and `null` will never make it to the top of it
- Remove `flushToResultSet` between `addReferenceVariantElements` calls because flush already happens inside of them
- ^KT-25732 Fixed

idea/idea-completion/src/org/jetbrains/kotlin/idea/completion/BasicCompletionSession.kt

@@ -237,6 +237,8 @@ class BasicCompletionSession(
                 ).completeDslFunctions()
             }
 
+            KEYWORDS_ONLY.doComplete()
+
             val contextVariableTypesForSmartCompletion = withCollectRequiredContextVariableTypes(::completeWithSmartCompletion)
 
             val contextVariableTypesForReferenceVariants = withCollectRequiredContextVariableTypes { lookupElementFactory ->
@@ -249,21 +251,17 @@ class BasicCompletionSession(
 
                     prefix[0].isLowerCase() -> {
                         addReferenceVariantElements(lookupElementFactory, USUALLY_START_LOWER_CASE.intersect(descriptorKindFilter))
-                        flushToResultSet()
                         addReferenceVariantElements(lookupElementFactory, USUALLY_START_UPPER_CASE.intersect(descriptorKindFilter))
                     }
 
                     else -> {
                         addReferenceVariantElements(lookupElementFactory, USUALLY_START_UPPER_CASE.intersect(descriptorKindFilter))
-                        flushToResultSet()
                         addReferenceVariantElements(lookupElementFactory, USUALLY_START_LOWER_CASE.intersect(descriptorKindFilter))
                     }
                 }
                 referenceVariantsCollector!!.collectingFinished()
             }
 
-            KEYWORDS_ONLY.doComplete()
-
             // getting root packages from scope is very slow so we do this in alternative way
             if (callTypeAndReceiver.receiver == null && callTypeAndReceiver.callType.descriptorKindFilter.kindMask.and(DescriptorKindFilter.PACKAGES_MASK) != 0) {
                 //TODO: move this code somewhere else?