Move to new Travis CI Dart task model

Loosing coveralls, but we can bring that back later once pkg/test
supports it

Author: kevmoo

.travis.yml

@@ -1,12 +1,26 @@
-sudo: false
-
 language: dart
+sudo: false
 dart:
-  - stable
   - dev
-
+  - stable
+  - 1.22.1
+  - 1.21.1
+  - 1.20.1
+  - 1.19.1
 cache:
   directories:
     - $HOME/.pub-cache
-
-script: ./tool/travis.sh
+dart_task:
+  # Run the tests -- include the default-skipped presubmit tests
+  - test: --platform vm --run-skipped
+matrix:
+  include:
+    # Only validate formatting using the dev release
+    # Formatted with 1.23.0-dev.10.0 which has (good) changes since 1.22.1
+    - dart: dev
+      dart_task: dartfmt
+    # Only care about being analyzer clean for dev and stable
+    - dart: dev
+      dart_task: dartanalyzer
+    - dart: stable
+      dart_task: dartanalyzer

README.md

@@ -1,5 +1,4 @@
 [![Build Status](https://travis-ci.org/dart-lang/source_gen.svg?branch=master)](https://travis-ci.org/dart-lang/source_gen)
-[![Coverage Status](https://coveralls.io/repos/dart-lang/source_gen/badge.svg?branch=master)](https://coveralls.io/r/dart-lang/source_gen)
 
 `source_gen` provides: