Emit warning on import of dart:mirrors.

sdk/lib/_internal/compiler/implementation/apiimpl.dart

Index: sdk/lib/_internal/compiler/implementation/apiimpl.dart
diff --git a/sdk/lib/_internal/compiler/implementation/apiimpl.dart b/sdk/lib/_internal/compiler/implementation/apiimpl.dart
index 8cb14ddc4d69fafe17373c707ba28f71fe2a1ed0..362d91c8ba453df50edf226dff7aaa263e831941 100644
--- a/sdk/lib/_internal/compiler/implementation/apiimpl.dart
+++ b/sdk/lib/_internal/compiler/implementation/apiimpl.dart
@@ -72,7 +72,9 @@ class Compiler extends leg.Compiler {
                 hasOption(options, '--show-package-warnings'),
             useContentSecurityPolicy: hasOption(options, '--csp'),
             hasIncrementalSupport: hasOption(options, '--incremental-support'),
-            suppressWarnings: hasOption(options, '--suppress-warnings')) {
+            suppressWarnings: hasOption(options, '--suppress-warnings'),
+            enableExperimentalMirrors:
+                hasOption(options, '--enable-experimental-mirrors')) {
     tasks.addAll([
         userHandlerTask = new leg.GenericTask('Diagnostic handler', this),
         userProviderTask = new leg.GenericTask('Input provider', this),