add isKotlinSourceElement method to Source interface in order to recognize kotlin source elements during compilation

compiler/frontend.java/src/org/jetbrains/kotlin/load/java/components/JavaSourceElementFactoryImpl.kt

@@ -24,6 +24,8 @@ import org.jetbrains.kotlin.load.java.sources.JavaSourceElementFactory
 import org.jetbrains.kotlin.load.java.sources.JavaSourceElement
 
 private class JavaSourceElementImpl(override val javaElement: JavaElement) : PsiSourceElement, JavaSourceElement {
+    override fun isKotlinSourceElement(): Boolean = false
+
     override val psi: PsiElement?
         get() = (javaElement as JavaElementImpl<*>).getPsi()
 }

compiler/frontend/src/org/jetbrains/kotlin/resolve/source/KotlinSourceElement.kt

@@ -20,7 +20,9 @@ import org.jetbrains.kotlin.psi.JetElement
 import org.jetbrains.kotlin.descriptors.SourceElement
 import com.intellij.psi.PsiElement
 
-public class KotlinSourceElement(override val psi: JetElement) : PsiSourceElement
+public class KotlinSourceElement(override val psi: JetElement) : PsiSourceElement {
+    override fun isKotlinSourceElement(): Boolean = true
+}
 
 public fun JetElement?.toSourceElement(): SourceElement = if (this == null) SourceElement.NO_SOURCE else KotlinSourceElement(this)