Remove upper case constants (#17)

* Remove usage of upper-case constants.

* update SDK version
* Remove 1.23.0 from travis config.
diff --git a/.travis.yml b/.travis.yml
index 8239344..c724bab 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -2,8 +2,6 @@
 
 dart:
   - dev
-  - stable
-  - 1.23.0
 
 dart_task:
   - test: --platform vm
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 717cbe9..11e0c50 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,6 +1,6 @@
 ## 3.1.2
 
-* Require Dart SDK 1.23 or greater.
+* Require Dart SDK 2.0.0-dev.17.0 or greater.
 
 * A number of strong-mode fixes.
 
diff --git a/pubspec.yaml b/pubspec.yaml
index 2a5fceb..b33887c 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -5,7 +5,7 @@
 description: >
   A platform-independent package for parsing and serializing HTTP formats.
 environment:
-  sdk: ">=1.23.0 <2.0.0"
+  sdk: ">=2.0.0-dev.17.0 <2.0.0"
 dependencies:
   charcode: "^1.1.0"
   collection: ">=0.9.1 <2.0.0"
diff --git a/test/http_date_test.dart b/test/http_date_test.dart
index 02b7c6f..887a2c4 100644
--- a/test/http_date_test.dart
+++ b/test/http_date_test.dart
@@ -43,7 +43,7 @@
       test("parses the example date", () {
         var date = parseHttpDate("Sun, 06 Nov 1994 08:49:37 GMT");
         expect(date.day, equals(6));
-        expect(date.month, equals(DateTime.NOVEMBER));
+        expect(date.month, equals(DateTime.november));
         expect(date.year, equals(1994));
         expect(date.hour, equals(8));
         expect(date.minute, equals(49));
@@ -147,7 +147,7 @@
       test("parses the example date", () {
         var date = parseHttpDate("Sunday, 06-Nov-94 08:49:37 GMT");
         expect(date.day, equals(6));
-        expect(date.month, equals(DateTime.NOVEMBER));
+        expect(date.month, equals(DateTime.november));
         expect(date.year, equals(1994));
         expect(date.hour, equals(8));
         expect(date.minute, equals(49));
@@ -239,7 +239,7 @@
       test("parses the example date", () {
         var date = parseHttpDate("Sun Nov  6 08:49:37 1994");
         expect(date.day, equals(6));
-        expect(date.month, equals(DateTime.NOVEMBER));
+        expect(date.month, equals(DateTime.november));
         expect(date.year, equals(1994));
         expect(date.hour, equals(8));
         expect(date.minute, equals(49));
@@ -250,7 +250,7 @@
       test("parses a date with a two-digit day", () {
         var date = parseHttpDate("Sun Nov 16 08:49:37 1994");
         expect(date.day, equals(16));
-        expect(date.month, equals(DateTime.NOVEMBER));
+        expect(date.month, equals(DateTime.november));
         expect(date.year, equals(1994));
         expect(date.hour, equals(8));
         expect(date.minute, equals(49));