Emit warning on import of dart:mirrors.

pkg/compiler/lib/src/apiimpl.dart

Index: pkg/compiler/lib/src/apiimpl.dart
diff --git a/pkg/compiler/lib/src/apiimpl.dart b/pkg/compiler/lib/src/apiimpl.dart
index 734bba32083c27f70c1b9f63e4c68e270c65e420..9b923bb1ad2d53591a8e9d0ef6fa842b34a67259 100644
--- a/pkg/compiler/lib/src/apiimpl.dart
+++ b/pkg/compiler/lib/src/apiimpl.dart
@@ -80,6 +80,8 @@ class Compiler extends leg.Compiler {
                 forceIncrementalSupport ||
                 hasOption(options, '--incremental-support'),
             suppressWarnings: hasOption(options, '--suppress-warnings'),
+            enableExperimentalMirrors:

[rmacnak, 2015/01/14 18:30:29]

Uhh, what do you intend to put behind this flag?

[floitsch, 2015/01/15 10:43:52]

The implementation of dart:mirrors in dart2js is gated by this flag.

[rmacnak, 2015/01/15 21:48:52]

The existing implementation? That's quite a significant breaking change.

[floitsch, 2015/01/15 22:35:15]

If adding a flag to a compilation process is a significant breaking change, then
yes.
Fwiw it's not going to happen in 1.9.

+                hasOption(options, '--enable-experimental-mirrors'),
             enableAsyncAwait: hasOption(options, '--enable-async'),
             enableEnums: hasOption(options, '--enable-enum')) {
     tasks.addAll([