Update the SDK constraint. (flutter#15)

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

Author: nex3

pubspec.yaml

@@ -7,6 +7,6 @@ dependencies:
   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'