Add Folder.getChildAssumingFile().

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 143e7191e80bcfc0b365e7dc27647f42f4be94f8..f95d33ffc1bb44bcc8c197cdfd5cdad448e43e19 100644
--- a/pkg/analyzer/lib/file_system/memory_file_system.dart
+++ b/pkg/analyzer/lib/file_system/memory_file_system.dart
@@ -498,6 +498,16 @@ class _MemoryFolder extends _MemoryResource implements Folder {
   }
 
   @override
+  _MemoryFile getChildAssumingFile(String relPath) {
+    String childPath = canonicalizePath(relPath);
+    _MemoryResource resource = _provider._pathToResource[childPath];
+    if (resource is _MemoryFile) {
+      return resource;
+    }
+    return new _MemoryFile(_provider, childPath);
+  }
+
+  @override
   _MemoryFolder getChildAssumingFolder(String relPath) {
     String childPath = canonicalizePath(relPath);
     _MemoryResource resource = _provider._pathToResource[childPath];