warn on file modified during build

Author: dcharkes

pkgs/native_assets_builder/lib/src/build_runner/build_runner.dart

@@ -540,7 +540,7 @@ ${e.message}
           }
         } else {
           cache.reset();
-          await cache.hashFiles(
+          final modifiedDuringBuild = await cache.hashFiles(
             [
               ...result.dependencies,
               // Also depend on the hook source code.
@@ -549,6 +549,9 @@ ${e.message}
             validBeforeLastModified: cacheCutoffTime,
           );
           await cache.persist();
+          if (modifiedDuringBuild != null) {
+            logger.severe('File modified during build. Build must be rerun.');
+          }
         }
         return result;
       },
@@ -717,11 +720,14 @@ ${e.message}
             .map(Uri.file)
             .toList();
         cache.reset();
-        await cache.hashFiles(
+        final modifiedDuringBuild = await cache.hashFiles(
           dartSources,
           validBeforeLastModified: cacheCutoffTime,
         );
         await cache.persist();
+        if (modifiedDuringBuild != null) {
+          logger.severe('File modified during build. Build must be rerun.');
+        }
       } else {
         await cacheFile.delete();
       }

pkgs/native_assets_builder/lib/src/file_system_cache/file_system_cache.dart

@@ -35,17 +35,19 @@ class FileSystemCache {
   ///
   /// If [validBeforeLastModified] is provided, any entities that were modified
   /// after [validBeforeLastModified] will get a dummy hash so that they will
-  /// show up as outdated.
-  Future<void> hashFiles(
+  /// show up as outdated. If any such entity exists, its uri will be returned.
+  Future<Uri?> hashFiles(
     List<Uri> fileSystemEntities, {
     DateTime? validBeforeLastModified,
   }) async {
+    Uri? modifiedAfterTimeStamp;
     for (final uri in fileSystemEntities) {
       int hash;
       if (validBeforeLastModified != null &&
           (await uri.fileSystemEntity.lastModified())
               .isAfter(validBeforeLastModified)) {
         hash = _hashLastModifiedAfterCutoff;
+        modifiedAfterTimeStamp = uri;
       } else {
         if (_isDirectoryPath(uri.path)) {
           hash = await _hashDirectory(uri);
@@ -55,6 +57,7 @@ class FileSystemCache {
       }
       _hashes.files.add(FilesystemEntityHash(uri, hash));
     }
+    return modifiedAfterTimeStamp;
   }
 
   Future<void> persist() =>