Dart 1.22 added a new keyword: covariant. http://news.dartlang.org/2017/02/dart-122-faster-tools-assert-messages.html

Closes #3878

`@ng1.Injectable` is a legal annotation

`import "package:foo/foo.dart" as bar;` should highlight "as" as a keyword.

In a raw string, a character following a "\" is not escaped.

Issue #3601

- Previously, triple-quoted multi-line strings could be closed with a
  single quote, e.g. CodeMirror thought the following was a legal string
  literal: '''This string literal is not terminated correctly'.
  Also reported here: dart-lang/dart-pad#667
- String interpolation with $identifier and ${expression} now works

Issue #2952