Facet serialization changed in order to keep compatibility with previous

versions of Kotlin plugin

core/descriptors/src/org/jetbrains/kotlin/platform/platformUtil.kt

@@ -25,10 +25,12 @@ fun <T> TargetPlatform?.has(klass: Class<T>): Boolean = this != null && subplatf
  * also provides better description for multiplatforms.
  */
 val TargetPlatform.oldFashionedDescription: String
-    get() = if (this.isCommon()) "Common (experimental) " else this.single().oldFashionedDescription
+    // the invocation if isCommon is not preferable as it could be governed by separate flag and may
+    // to appear independently on count of target platforms
+    get() = this.singleOrNull()?.oldFashionedDescription ?: "Common (experimental) "
 
 
-/**
+    /**
  * Renders multiplatform in form
  *      '$PLATFORM_1 / $PLATFORM_2 / ...'
  * e.g.

idea/idea-jps-common/src/org/jetbrains/kotlin/config/facetSerialization.kt

@@ -296,6 +296,7 @@ private fun KotlinFacetSettings.writeLatestConfig(element: Element) {
     val filter = SkipDefaultsSerializationFilter()
 
     targetPlatform?.let {
+        element.setAttribute("platform", it.oldFashionedDescription)
         element.setAttribute("allPlatforms", it.componentPlatforms.map { it.serializeToString() }.joinToString(separator = "/"))
     }
     if (!useProjectSettings) {

idea/testData/configuration/loadAndSaveProjectWithV2FacetConfig/module.iml

@@ -2,7 +2,7 @@
 <module type="JAVA_MODULE" version="4">
   <component name="FacetManager">
     <facet type="kotlin-language" name="Kotlin">
-      <configuration version="2" allPlatforms="JVM [1.8]" useProjectSettings="false">
+      <configuration version="2" platform="JVM 1.8" allPlatforms="JVM [1.8]" useProjectSettings="false">
         <compilerSettings>
           <option name="additionalArguments" value="-version -Xallow-kotlin-package -Xskip-metadata-version-check" />
         </compilerSettings>

idea/tests/org/jetbrains/kotlin/idea/configuration/ConfigureKotlinInTempDirTest.kt

@@ -103,7 +103,7 @@ open class ConfigureKotlinInTempDirTest : AbstractConfigureKotlinInTempDirTest()
         application.isSaveAllowed = true
         application.saveAll()
         val moduleFileContentAfter = String(module.moduleFile!!.contentsToByteArray())
-        Assert.assertEquals(moduleFileContentBefore.replace("platform=\"JVM 1.8\"", "allPlatforms=\"JVM [1.8]\""), moduleFileContentAfter)
+        Assert.assertEquals(moduleFileContentBefore.replace("platform=\"JVM 1.8\"", "platform=\"JVM 1.8\" allPlatforms=\"JVM [1.8]\""), moduleFileContentAfter)
     }
 
     fun testApiVersionWithoutLanguageVersion() {