Update travis-CI to use dev (dart-lang/source_span#58)

pkgs/source_span/.travis.yml

diff --git a/pkgs/source_span/.travis.yml b/pkgs/source_span/.travis.yml
index 95f19ae..8a4761a 100644
--- a/pkgs/source_span/.travis.yml
+++ b/pkgs/source_span/.travis.yml
@@ -1,28 +1,24 @@
 language: dart
 
 dart:
-- preview/raw/2.10.0-0.2-dev
+- dev
 
 jobs:
   include:
     - stage: analyze_and_format
       name: "Analyze test/"
-      dart: preview/raw/2.10.0-0.2-dev
       os: linux
       script: dartanalyzer --enable-experiment=non-nullable --fatal-warnings --fatal-infos .
     - stage: analyze_and_format
       name: "Format"
-      dart: preview/raw/2.10.0-0.2-dev
       os: linux
       script: dartfmt -n --set-exit-if-changed .
     - stage: test
       name: "Vm Tests"
-      dart: preview/raw/2.10.0-0.2-dev
       os: linux
       script: pub run --enable-experiment=non-nullable test -p vm
     - stage: test
       name: "Web Tests"
-      dart: preview/raw/2.10.0-0.2-dev
       os: linux
       script: pub run --enable-experiment=non-nullable test -p chrome