Support latest built_collection (#185)

Fixes https://github.com/dart-lang/code_builder/issues/184

Author: kevmoo

pubspec.yaml

@@ -12,7 +12,7 @@ web:
     debug: dartdevc
 
 dependencies:
-  built_collection: '>=1.0.0 <3.0.0'
+  built_collection: '>=1.0.0 <4.0.0'
   built_value: '>=2.0.0 <5.0.0'
   matcher: ^0.12.0
   meta: ^1.0.5