Add preview_dart_2 to supported set of status file variables

R: [email protected]
Change-Id: I9d77800f7db42207c5e730706b02b38080e85299
Reviewed-on: https://dart-review.googlesource.com/45322
Reviewed-by: Paul Berry <[email protected]>

Author: whesse

tools/testing/dart/environment.dart

@@ -32,6 +32,7 @@ final _variables = {
   "jscl": new _Variable.bool((c) => c.runtime.isJSCommandLine),
   "minified": new _Variable.bool((c) => c.isMinified),
   "mode": new _Variable((c) => c.mode.name, Mode.names),
+  "preview_dart_2": new _Variable.bool((c) => c.previewDart2),
   "runtime": new _Variable(_runtimeName, Runtime.names),
   "spec_parser": new _Variable.bool((c) => c.compiler == Compiler.specParser),
   "strong": new _Variable.bool((c) => c.isStrong),