Update the SDK constraint.

This allows us to use ^-style constraints.

R=kevmoo@google.com

Review URL: https://codereview.chromium.org//1763533002 .

pkgs/web_socket_channel/pubspec.yaml

diff --git a/pkgs/web_socket_channel/pubspec.yaml b/pkgs/web_socket_channel/pubspec.yaml
index 2888da6..f7b58e5 100644
--- a/pkgs/web_socket_channel/pubspec.yaml
+++ b/pkgs/web_socket_channel/pubspec.yaml
@@ -5,7 +5,7 @@
 homepage: https://github.com/dart-lang/web_socket_channel
 
 environment:
-  sdk: '>=1.0.0 <2.0.0'
+  sdk: '>=1.8.0 <2.0.0'
 
 dependencies:
   async: '^1.3.0'