dart2js: Fix check for support of reflection.

pkg/compiler/lib/src/compiler.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of dart2js;
 
 /**
  * If true, print a warning for each method that was resolved, but not
  * compiled.
  */
@@ skipping 237 matching lines @@
   void registerInstantiation(InterfaceType type);
 
   void registerGetOfStaticFunction(FunctionElement element);
 }
 
 abstract class Backend {
   final Compiler compiler;
 
   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;
 
   /// The constant environment for the backend interpretation of compile-time
   /// constants.
   BackendConstantEnvironment get constants;
 
   /// The compiler task responsible for the compilation of constants for both
   /// the frontend and the backend.
@@ skipping 1094 matching lines @@
 
           chainCount++;
           if (chainCount > chainLimit) {
             // Assume there are more import chains.
             importChains.add('...');
             return false;
           }
           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,
               {'importChain': importChains.join(
                    MessageKind.IMPORT_EXPERIMENTAL_MIRRORS_PADDING)});
         }
       }
 
       functionClass.ensureResolved(this);
       functionApplyMethod = functionClass.lookupLocalMember('apply');
@@ skipping 1111 matching lines @@
   InterfaceType streamType([DartType elementType]) {
     InterfaceType type = streamClass.computeType(compiler);
     if (elementType == null) {
       return streamClass.rawType;
     }
     return type.createInstantiation([elementType]);
   }
 }
 
 typedef void InternalErrorFunction(Spannable location, String message);