Remove upper case constants (flutter#6)

* Remove usage of upper-case constants.
* update SDK version
* remove stable from Travis config
* Update pubspec.yaml

Author: lrhn

.travis.yml

@@ -2,8 +2,6 @@ language: dart
 sudo: false
 dart:
   - dev
-  - stable
-
 dart_task:
   - test
   - dartfmt

pubspec.yaml

@@ -1,10 +1,10 @@
 name: http_throttle
-version: 1.0.2-dev
+version: 1.0.2
 author: Dart Team <[email protected]>
 homepage: https://github.com/dart-lang/http_throttle
 description: HTTP client middleware that throttles requests.
 environment:
-  sdk: ">=1.13.0 <2.0.0"
+  sdk: ">=2.0.0-dev.17.0 <2.0.0"
 dependencies:
   http: ">=0.9.0 <0.12.0"
   pool: ">=1.0.0 <2.0.0"

test/http_throttle_test.dart

@@ -76,5 +76,5 @@ Future pumpEventQueue([int times = 20]) {
   // Future.value or Future() constructors use scheduleMicrotask themselves and
   // would therefore not wait for microtask callbacks that are scheduled after
   // invoking this method.
-  return new Future.delayed(Duration.ZERO, () => pumpEventQueue(times - 1));
+  return new Future.delayed(Duration.zero, () => pumpEventQueue(times - 1));
 }