Add Resource.delete() method.

pkg/analyzer/lib/file_system/physical_file_system.dart

Index: pkg/analyzer/lib/file_system/physical_file_system.dart
diff --git a/pkg/analyzer/lib/file_system/physical_file_system.dart b/pkg/analyzer/lib/file_system/physical_file_system.dart
index 132375f34bd9e79e332fd0f6a6a5397b7d154f85..9cd62b6b89ed269e9d1ad5b181643e29bbcb3d3a 100644
--- a/pkg/analyzer/lib/file_system/physical_file_system.dart
+++ b/pkg/analyzer/lib/file_system/physical_file_system.dart
@@ -263,5 +263,14 @@ abstract class _PhysicalResource implements Resource {
   }
 
   @override
+  void delete() {
+    try {
+      _entry.deleteSync(recursive: true);
+    } on io.FileSystemException catch (exception) {
+      throw new FileSystemException(exception.path, exception.message);
+    }
+  }
+
+  @override
   String toString() => path;
 }