Update the SDK constraint. (dart-lang/source_span#15)

If we're using ^ constraints, we need a tighter SDK constraint.

pkgs/source_span/pubspec.yaml

diff --git a/pkgs/source_span/pubspec.yaml b/pkgs/source_span/pubspec.yaml
index d340d85..70f4e10 100644
--- a/pkgs/source_span/pubspec.yaml
+++ b/pkgs/source_span/pubspec.yaml
@@ -7,6 +7,6 @@
   charcode: '^1.0.0'
   path: '>=1.2.0 <2.0.0'
 environment:
-  sdk: '>=0.8.10+6 <2.0.0'
+  sdk: '>=1.8.0 <2.0.0'
 dev_dependencies:
   test: '>=0.12.0 <0.13.0'