Save in an isolate when available

 # Changelog
 
+## 3.11.0
+
+- Save in an isolate when available
+
 ## 3.10.8
 
 - Add Flutter's Logical Pixel constant

@@ -251,11 +251,13 @@ class PdfDocument {
 
   /// Generate the PDF document as a memory file
   Future<Uint8List> save() async {
-    final os = PdfStream();
-    if (prev != null) {
-      os.putBytes(prev!.bytes);
-    }
-    await _write(os);
-    return os.output();
+    return pdfCompute(() async {
+      final os = PdfStream();
+      if (prev != null) {
+        os.putBytes(prev!.bytes);
+      }
+      await _write(os);
+      return os.output();
+    });
   }
 }

@@ -20,3 +20,10 @@ import '../format/object_base.dart';
 
 /// Zip compression function
 DeflateCallback defaultDeflate = const ZLibEncoder().encode;
+
+/// The dart:html implementation of [pdfCompute].
+@pragma('dart2js:tryInline')
+Future<R> pdfCompute<R>(Future<R> Function() computation) async {
+  await null;
+  return computation();
+}

@@ -15,8 +15,14 @@
  */
 
 import 'dart:io';
+import 'dart:isolate';
 
 import '../format/object_base.dart';
 
 /// Zip compression function
 DeflateCallback defaultDeflate = zlib.encode;
+
+/// The dart:io implementation of [pdfCompute].
+@pragma('vm:prefer-inline')
+Future<R> pdfCompute<R>(Future<R> Function() computation) async =>
+    Isolate.run<R>(computation, debugName: 'dart_pdf');

@@ -15,7 +15,7 @@ topics:
 version: 3.10.8
 
 environment:
-  sdk: ">=2.18.0 <4.0.0"
+  sdk: ">=2.19.0 <4.0.0"
 
 dependencies:
   archive: ^3.1.0