Increase the SDK constraint to allow ^ constraints.

R=rnystrom@google.com

Review URL: https://codereview.chromium.org//1325943002 .

pkgs/string_scanner/pubspec.yaml

diff --git a/pkgs/string_scanner/pubspec.yaml b/pkgs/string_scanner/pubspec.yaml
index efc7b0a..3e71eb7 100644
--- a/pkgs/string_scanner/pubspec.yaml
+++ b/pkgs/string_scanner/pubspec.yaml
@@ -6,9 +6,9 @@
   A class for parsing strings using a sequence of patterns.
 dependencies:
   charcode: "^1.1.0"
-  path: ">=1.2.0 <2.0.0"
-  source_span: ">=1.0.0 <2.0.0"
+  path: "^1.2.0"
+  source_span: "^1.0.0"
 dev_dependencies:
   test: ">=0.12.0 <0.13.0"
 environment:
-  sdk: ">=1.2.0 <2.0.0"
+  sdk: ">=1.8.0 <2.0.0"