support the latest pkg:build_web_compilers (dart-lang/markdown#250)

pkgs/markdown/pubspec.yaml

diff --git a/pkgs/markdown/pubspec.yaml b/pkgs/markdown/pubspec.yaml
index 7ec2c5e..6289b39 100644
--- a/pkgs/markdown/pubspec.yaml
+++ b/pkgs/markdown/pubspec.yaml
@@ -18,7 +18,7 @@
 dev_dependencies:
   build_runner: ^1.0.0
   build_version: ^2.0.0
-  build_web_compilers: ^1.0.0
+  build_web_compilers: '>=1.0.0 <3.0.0'
   collection: ^1.2.0
   expected_output: ^1.2.1
   html: '>=0.12.2 <0.15.0'