Ignore non-existent analysis roots.

pkg/analysis_server/lib/src/context_manager.dart

Index: pkg/analysis_server/lib/src/context_manager.dart
diff --git a/pkg/analysis_server/lib/src/context_manager.dart b/pkg/analysis_server/lib/src/context_manager.dart
index ac5bceda2a07c2cd88a26b77f75f5bad2c838958..8f079cc50a59e743e0e0005476c709d96827e41f 100644
--- a/pkg/analysis_server/lib/src/context_manager.dart
+++ b/pkg/analysis_server/lib/src/context_manager.dart
@@ -531,6 +531,10 @@ class ContextManagerImpl implements ContextManager {
       Resource resource = resourceProvider.getResource(path);
       if (resource is Folder) {
         includedFolders.add(resource);
+      } else if (!resource.exists) {
+        // Non-existent resources are ignored.  TODO(paulberry): we should set
+        // up a watcher to ensure that if the resource appears later, we will
+        // begin analyzing it.
       } else {
         // TODO(scheglov) implemented separate files analysis
         throw new UnimplementedError('$path is not a folder. '