Enable and fix lint prefer_collection_literals (#310)

Requires bumping the SDK to use set literals. This also fixes some
annoyances with the analyzer in older SDK versions.

Author: natebosch

.travis.yml

@@ -2,7 +2,7 @@ language: dart
 
 dart:
   - dev
-  - 2.0.0
+  - 2.2.0
 
 dart_task:
   - test: --platform vm

analysis_options.yaml

@@ -48,6 +48,7 @@ linter:
   - prefer_contains
   - prefer_equal_for_default_values
   - prefer_final_fields
+  - prefer_collection_literals
   - prefer_generic_function_type_aliases
   - prefer_initializing_formals
   - prefer_is_empty

lib/src/browser_client.dart

@@ -29,7 +29,7 @@ class BrowserClient extends BaseClient {
   /// The currently active XHRs.
   ///
   /// These are aborted if the client is closed.
-  final _xhrs = Set<HttpRequest>();
+  final _xhrs = <HttpRequest>{};
 
   /// Creates a new HTTP client.
   BrowserClient();

pubspec.yaml

@@ -5,7 +5,7 @@ homepage: https://github.com/dart-lang/http
 description: A composable, multi-platform, Future-based API for HTTP requests.
 
 environment:
-  sdk: ">=2.0.0 <3.0.0"
+  sdk: ">=2.2.0 <3.0.0"
 
 dependencies:
   async: ">=1.10.0 <3.0.0"