Refactor skipMetadataVersionCheck flag

To make addition of other flags easier in the future, provide a more
abstract 'isFlagEnabled' in LanguageVersionSettings

compiler/util/src/org/jetbrains/kotlin/config/AnalysisFlags.kt

@@ -0,0 +1,42 @@
+/*
+ * Copyright 2010-2017 JetBrains s.r.o.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.jetbrains.kotlin.config
+
+import kotlin.properties.ReadOnlyProperty
+import kotlin.reflect.KProperty
+
+class AnalysisFlag internal constructor(private val name: String) {
+    override fun equals(other: Any?): Boolean = other is AnalysisFlag && other.name == name
+
+    override fun hashCode(): Int = name.hashCode()
+
+    override fun toString(): String = name
+
+    companion object
+}
+
+private operator fun AnalysisFlag.Companion.provideDelegate(instance: Any?, property: KProperty<*>) =
+        object : ReadOnlyProperty<Any?, AnalysisFlag> {
+            private val flag = AnalysisFlag(property.name)
+
+            override fun getValue(thisRef: Any?, property: KProperty<*>): AnalysisFlag = flag
+        }
+
+object AnalysisFlags {
+    @JvmStatic
+    val skipMetadataVersionCheck by AnalysisFlag
+}

compiler/util/src/org/jetbrains/kotlin/config/LanguageVersionSettings.kt

@@ -106,20 +106,32 @@ interface LanguageVersionSettings {
     fun supportsFeature(feature: LanguageFeature): Boolean =
             getFeatureSupport(feature).let { it == LanguageFeature.State.ENABLED || it == LanguageFeature.State.ENABLED_WITH_WARNING }
 
+    fun isFlagEnabled(flag: AnalysisFlag): Boolean
+
     val apiVersion: ApiVersion
 
     // Please do not use this to enable/disable specific features/checks. Instead add a new LanguageFeature entry and call supportsFeature
     val languageVersion: LanguageVersion
-
-    val skipMetadataVersionCheck: Boolean
 }
 
 class LanguageVersionSettingsImpl @JvmOverloads constructor(
         override val languageVersion: LanguageVersion,
         override val apiVersion: ApiVersion,
-        override val skipMetadataVersionCheck: Boolean = false,
         private val specificFeatures: Map<LanguageFeature, LanguageFeature.State> = emptyMap()
 ) : LanguageVersionSettings {
+    private val enabledFlags = hashSetOf<AnalysisFlag>()
+
+    override fun isFlagEnabled(flag: AnalysisFlag): Boolean = flag in enabledFlags
+
+    fun switchFlag(flag: AnalysisFlag, enable: Boolean) {
+        if (enable) {
+            enabledFlags.add(flag)
+        }
+        else {
+            enabledFlags.remove(flag)
+        }
+    }
+
     override fun getFeatureSupport(feature: LanguageFeature): LanguageFeature.State {
         specificFeatures[feature]?.let { return it }