Update travis-ci: test on dev (dart-lang/source_maps#52)

pkgs/source_maps/.travis.yml

diff --git a/pkgs/source_maps/.travis.yml b/pkgs/source_maps/.travis.yml
index 51a3d8a..ac74df9 100644
--- a/pkgs/source_maps/.travis.yml
+++ b/pkgs/source_maps/.travis.yml
@@ -1,23 +1,20 @@
 language: dart
 
 dart:
- - preview/raw/2.10.0-0.2-dev
+ - dev
 
 jobs:
   include:
     - stage: analyze_and_format
       name: "Analyze"
-      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