Migrate off deprecated isInstanceOf (#40)

- Bump `package:test` to `1.6.0` which introduce `isA`.
- Use `2.1.0` SDK on travis since it's the minimum supported by `test`.
- Bump min SDK in the pubspec to `2.1.0` to match what is tested.
- Replace `new isInstanceOf` with `isA`.

.travis.yml

@@ -2,7 +2,7 @@ language: dart
 
 dart:
   - dev
-  - 2.0.0
+  - 2.1.0
 
 dart_task:
   - test: --platform vm,chrome

pubspec.yaml

@@ -6,12 +6,12 @@ author: Dart Team <[email protected]>
 homepage: https://github.com/dart-lang/source_span
 
 environment:
-  sdk: '>=2.0.0 <3.0.0'
+  sdk: '>=2.1.0 <3.0.0'
 
 dependencies:
   charcode: ^1.0.0
   path: '>=1.2.0 <2.0.0'
   term_glyph: ^1.0.0
 
 dev_dependencies:
-  test: ^1.0.0
+  test: ^1.6.0

test/file_test.dart

@@ -241,7 +241,7 @@ zip zap zop""", url: "bar.dart").span(10, 11);
     test("pointSpan() returns a FileSpan", () {
       var location = file.location(15);
       var span = location.pointSpan();
-      expect(span, new isInstanceOf<FileSpan>());
+      expect(span, isA<FileSpan>());
       expect(span.start, equals(location));
       expect(span.end, equals(location));
       expect(span.text, isEmpty);
@@ -344,14 +344,14 @@ zip zap zop
       });
 
       test("returns a FileSpan for a FileSpan input", () {
-        expect(span.union(file.span(0, 5)), new isInstanceOf<FileSpan>());
+        expect(span.union(file.span(0, 5)), isA<FileSpan>());
       });
 
       test("returns a base SourceSpan for a SourceSpan input", () {
         var other = new SourceSpan(new SourceLocation(0, sourceUrl: "foo.dart"),
             new SourceLocation(5, sourceUrl: "foo.dart"), "hey, ");
         var result = span.union(other);
-        expect(result, isNot(new isInstanceOf<FileSpan>()));
+        expect(result, isNot(isA<FileSpan>()));
         expect(result.start, equals(other.start));
         expect(result.end, equals(span.end));
         expect(result.text, equals("hey, ar baz\n"));