More test clean-up

pkg/analyzer/lib/file_system/memory_file_system.dart

Index: pkg/analyzer/lib/file_system/memory_file_system.dart
diff --git a/pkg/analyzer/lib/file_system/memory_file_system.dart b/pkg/analyzer/lib/file_system/memory_file_system.dart
index a603eaa3c47897827634a8986c956d771e3a8bdb..44f8a062661923f70ce486d50f28fc451d42d679 100644
--- a/pkg/analyzer/lib/file_system/memory_file_system.dart
+++ b/pkg/analyzer/lib/file_system/memory_file_system.dart
@@ -236,6 +236,9 @@ class MemoryResourceProvider implements ResourceProvider {
   void _checkFileAtPath(String path) {
     _MemoryResource resource = _pathToResource[path];
     if (resource is! _MemoryFile) {
+      if (resource == null) {
+        throw new ArgumentError('File expected at "$path" but does not exist');
+      }
       throw new ArgumentError(
           'File expected at "$path" but ${resource.runtimeType} found');
     }