Replace making getInnermostContext public with making a getFolder method

pkg/analysis_server/lib/src/single_context_manager.dart

Index: pkg/analysis_server/lib/src/single_context_manager.dart
diff --git a/pkg/analysis_server/lib/src/single_context_manager.dart b/pkg/analysis_server/lib/src/single_context_manager.dart
index 0efb049d2823ec9477440b0174e077c35ed6da26..ed1d816350da9e76e2e665e327afafa94eb3e504 100644
--- a/pkg/analysis_server/lib/src/single_context_manager.dart
+++ b/pkg/analysis_server/lib/src/single_context_manager.dart
@@ -147,6 +147,14 @@ class SingleContextManager implements ContextManager {
   }
 
   @override
+  Folder getContextFolderFor(String path) {
+    if (isInAnalysisRoot(path)) {
+      return contextFolder;
+    }
+    return null;
+  }
+
+  @override
   AnalysisDriver getDriverFor(String path) {
     throw new UnimplementedError(
         'Unexpected invocation of getDriverFor in SingleContextManager');