dart2js: Fix check for support of reflection.

pkg/compiler/lib/src/compiler.dart

Index: pkg/compiler/lib/src/compiler.dart
diff --git a/pkg/compiler/lib/src/compiler.dart b/pkg/compiler/lib/src/compiler.dart
index 447be5fa202c2f117415961f4e563ae0d9fc773d..d35e5014312d3510a46c400781689bb9165c982b 100644
--- a/pkg/compiler/lib/src/compiler.dart
+++ b/pkg/compiler/lib/src/compiler.dart
@@ -255,6 +255,9 @@ abstract class Backend {
 
   Backend(this.compiler);
 
+  /// Returns true if the backend supports reflection.
+  bool get supportsReflection;
+
   /// The [ConstantSystem] used to interpret compile-time constants for this
   /// backend.
   ConstantSystem get constantSystem;
@@ -1369,9 +1372,9 @@ abstract class Compiler implements DiagnosticListener {
           return true;
         });
 
-        if (const bool.fromEnvironment("dart2js.use.new.emitter")) {
+        if (!backend.supportsReflection) {
           reportError(NO_LOCATION_SPANNABLE,
-                      MessageKind.MIRRORS_LIBRARY_NEW_EMITTER);
+                      MessageKind.MIRRORS_LIBRARY_NOT_SUPPORT_BY_BACKEND);
         } else {
           reportWarning(NO_LOCATION_SPANNABLE,
              MessageKind.IMPORT_EXPERIMENTAL_MIRRORS,