Address PR feedback

internal/zinc-core/src/main/scala/sbt/internal/inc/SourceInfo.scala

@@ -69,7 +69,7 @@ object SourceInfos {
   }
 }
 
-private[inc] final class MSourceInfos(val allInfos: Map[VirtualFileRef, SourceInfo])
+private final class MSourceInfos(val allInfos: Map[VirtualFileRef, SourceInfo])
     extends SourceInfos {
 
   def ++(o: SourceInfos) = {

internal/zinc-testing/src/main/scala/xsbti/TestCallback.scala

@@ -154,7 +154,7 @@ class TestCallback extends AnalysisCallback3 {
 
   override def getPickleJarPair = Optional.empty()
 
-  override def getSourceInfos: ReadSourceInfos = null
+  override def getSourceInfos: ReadSourceInfos = new TestSourceInfos
 }
 
 object TestCallback {

internal/zinc-testing/src/main/scala/xsbti/TestSourceInfo.scala

@@ -0,0 +1,23 @@
+/*
+ * Zinc - The incremental compiler for Scala.
+ * Copyright Scala Center, Lightbend, and Mark Harrah
+ *
+ * Licensed under Apache License 2.0
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * See the NOTICE file distributed with this work for
+ * additional information regarding copyright ownership.
+ */
+
+package xsbti
+
+import xsbti.compile.analysis.SourceInfo
+
+class TestSourceInfo extends SourceInfo {
+
+  override def getReportedProblems: Array[Problem] = Array.empty[Problem]
+
+  override def getUnreportedProblems: Array[Problem] = Array.empty[Problem]
+
+  override def getMainClasses: Array[String] = Array.empty[String]
+}

internal/zinc-testing/src/main/scala/xsbti/TestSourceInfos.scala

@@ -0,0 +1,24 @@
+/*
+ * Zinc - The incremental compiler for Scala.
+ * Copyright Scala Center, Lightbend, and Mark Harrah
+ *
+ * Licensed under Apache License 2.0
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * See the NOTICE file distributed with this work for
+ * additional information regarding copyright ownership.
+ */
+
+package xsbti
+
+import xsbti.compile.analysis.{ ReadSourceInfos, SourceInfo }
+
+import java.util
+
+class TestSourceInfos extends ReadSourceInfos {
+
+  override def get(sourceFile: VirtualFileRef): SourceInfo = new TestSourceInfo
+
+  override def getAllSourceInfos: util.Map[VirtualFileRef, SourceInfo] =
+    new util.HashMap[VirtualFileRef, SourceInfo]()
+}