Count kernel bytes in bytes synced by DevFS.update (#18288)

DevFS.update() returns the number of bytes synced from the host to the
target device. When running in Dart 2 mode, ensure that the size of the
kernel file is included in the count of bytes synced.

packages/flutter_tools/lib/src/devfs.dart

@@ -397,7 +397,9 @@ class DevFS {
     printTrace('DevFS: Deleted filesystem on the device ($_baseUri)');
   }
 
-  /// Update files on the device and return the number of bytes sync'd
+  /// Updates files on the device.
+  ///
+  /// Returns the number of bytes synced.
   Future<int> update({
     String mainPath,
     String target,
@@ -511,12 +513,13 @@ class DevFS {
         final String entryUri = projectRootPath != null ?
             fs.path.relative(mainPath, from: projectRootPath):
             mainPath;
-        dirtyEntries.putIfAbsent(
-          fs.path.toUri(entryUri + '.dill'),
-          () => new DevFSFileContent(fs.file(compiledBinary))
-        );
+        final Uri kernelUri = fs.path.toUri(entryUri + '.dill');
+        if (!dirtyEntries.containsKey(kernelUri)) {
+          final DevFSFileContent content = new DevFSFileContent(fs.file(compiledBinary));
+          dirtyEntries[kernelUri] = content;
+          numBytes += content.size;
+        }
       }
-
     }
     if (dirtyEntries.isNotEmpty) {
       printTrace('Updating files');