Update dependencies and switch to nullsafety (#24)

Due to the nature of the package the actual code is already null-safe.
The only changes necessary are unit tests
checking for throwing errors on null.
diff --git a/.travis.yml b/.travis.yml
index 60d1d5e..14549bb 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,7 +1,7 @@
 language: dart
 
 dart:
-  - 2.2.0
+  - 2.12.0-29.10.beta
   - dev
 
 dart_task:
diff --git a/pubspec.yaml b/pubspec.yaml
index f950a62..824b160 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -1,15 +1,15 @@
 name: tuple
-version: 1.0.4-dev
+version: 2.0.0-nullsafety.0
 
 description: Tuple data structure
 homepage: https://github.com/google/tuple.dart
 
 environment:
-  sdk: '>=2.2.0 <3.0.0'
+  sdk: '>=2.12.0-0 <3.0.0'
 
 dependencies:
-  quiver: '>=0.22.0 <3.0.0'
+  quiver: ^3.0.0-nullsafety
 
 dev_dependencies:
-  pedantic: ^1.2.0
-  test: ^1.0.0
+  pedantic: ^1.10.0-nullsafety
+  test: ^1.16.0-nullsafety
diff --git a/test/tuple_test.dart b/test/tuple_test.dart
index 006cdc4..d0f5032 100644
--- a/test/tuple_test.dart
+++ b/test/tuple_test.dart
@@ -17,10 +17,6 @@
     });
 
     group('\'s fromList', () {
-      test('throws when items is null', () {
-        expect(() => Tuple2<int, String>.fromList(null), throwsA(anything));
-      });
-
       test('throws when items is empty', () {
         expect(() => Tuple2<int, String>.fromList([]), throwsArgumentError);
       });
@@ -69,10 +65,6 @@
       expect('$t', '[1, a]');
     });
 
-    test('does not equal null', () {
-      expect(t == null, isFalse);
-    });
-
     test('does not equal when first value is different', () {
       expect(t == t.withItem1(2), isFalse);
     });
@@ -110,11 +102,6 @@
     });
 
     group('\'s fromList', () {
-      test('throws when items is null', () {
-        expect(
-            () => Tuple3<int, String, int>.fromList(null), throwsA(anything));
-      });
-
       test('throws when items is empty', () {
         expect(
             () => Tuple3<int, String, int>.fromList([]), throwsArgumentError);
@@ -180,10 +167,6 @@
       expect('$t', '[1, a, 10]');
     });
 
-    test('does not equal null', () {
-      expect(t == null, isFalse);
-    });
-
     test('does not equal when first value is different', () {
       expect(t == t.withItem1(2), isFalse);
     });
@@ -222,11 +205,6 @@
     });
 
     group('\'s fromList', () {
-      test('throws when items is null', () {
-        expect(() => Tuple4<int, String, int, String>.fromList(null),
-            throwsA(anything));
-      });
-
       test('throws when items is empty', () {
         expect(() => Tuple4<int, String, int, String>.fromList([]),
             throwsArgumentError);
@@ -313,10 +291,6 @@
       expect('$t', '[1, a, 10, b]');
     });
 
-    test('does not equal null', () {
-      expect(t == null, isFalse);
-    });
-
     test('does not equal when first value is different', () {
       expect(t == t.withItem1(2), isFalse);
     });
@@ -360,11 +334,6 @@
     });
 
     group('\'s fromList', () {
-      test('throws when items is null', () {
-        expect(() => Tuple5<int, String, int, String, int>.fromList(null),
-            throwsA(anything));
-      });
-
       test('throws when items is empty', () {
         expect(() => Tuple5<int, String, int, String, int>.fromList([]),
             throwsArgumentError);
@@ -478,10 +447,6 @@
       expect('$t', '[1, a, 10, b, 100]');
     });
 
-    test('does not equal null', () {
-      expect(t == null, isFalse);
-    });
-
     test('does not equal when first value is different', () {
       expect(t == t.withItem1(2), isFalse);
     });
@@ -533,12 +498,6 @@
     });
 
     group('\'s fromList', () {
-      test('throws when items is null', () {
-        expect(
-            () => Tuple6<int, String, int, String, int, String>.fromList(null),
-            throwsA(anything));
-      });
-
       test('throws when items is empty', () {
         expect(() => Tuple6<int, String, int, String, int, String>.fromList([]),
             throwsArgumentError);
@@ -687,10 +646,6 @@
       expect('$t', '[1, a, 10, b, 100, c]');
     });
 
-    test('does not equal null', () {
-      expect(t == null, isFalse);
-    });
-
     test('does not equal when first value is different', () {
       expect(t == t.withItem1(2), isFalse);
     });
@@ -752,13 +707,6 @@
     });
 
     group('\'s fromList', () {
-      test('throws when items is null', () {
-        expect(
-            () => Tuple7<int, String, int, String, int, String, int>.fromList(
-                null),
-            throwsA(anything));
-      });
-
       test('throws when items is empty', () {
         expect(
             () =>
@@ -931,10 +879,6 @@
       expect('$t', '[1, a, 10, b, 100, c, 1000]');
     });
 
-    test('does not equal null', () {
-      expect(t == null, isFalse);
-    });
-
     test('does not equal when first value is different', () {
       expect(t == t.withItem1(2), isFalse);
     });