Only support Dart 2

This is necessary because we're calling Set.cast() and Set.retype().
diff --git a/.travis.yml b/.travis.yml
index 63f5e5e..140ce59 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -2,16 +2,17 @@
 
 dart:
   - dev
-  - stable
 
 dart_task:
   - test
-  - dartanalyzer
 
 matrix:
   include:
     - dart: dev
       dart_task: dartfmt
+  include:
+    - dart: dev
+      dart_task: dartanalyzer
 
 # Only building master means that we don't run two builds for each pull request.
 branches:
diff --git a/pubspec.yaml b/pubspec.yaml
index e3c477c..a9a4cdb 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -9,4 +9,4 @@
 dev_dependencies:
   test: ">=0.12.0 <0.13.0"
 environment:
-  sdk: ">=1.0.0 <2.0.0"
+  sdk: ">=2.0.0-dev.35.0 <2.0.0"