Delete dart_backend from compiler.

tests/compiler/dart2js/message_kind_helper.dart

 // Copyright (c) 2013, 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.
 
 library dart2js.test.message_kind_helper;
 
 import 'package:expect/expect.dart';
 import 'dart:async';
 
 import 'package:compiler/src/commandline_options.dart';
 import 'package:compiler/src/compiler.dart' show
     Compiler;
-import 'package:compiler/src/dart_backend/dart_backend.dart' show
-    DartBackend;
 import 'package:compiler/src/diagnostics/messages.dart' show
     MessageKind,
     MessageTemplate;
 import 'package:compiler/compiler_new.dart' show
     Diagnostic;
 
 import 'memory_compiler.dart';
 
 const String ESCAPE_REGEXP = r'[[\]{}()*+?.\\^$|]';
 
@@ skipping 39 matching lines @@
     if (example is String) {
       example = {'main.dart': example};
     } else {
       Expect.isTrue(example is Map,
                     "Example must be either a String or a Map.");
       Expect.isTrue(example.containsKey('main.dart'),
                     "Example map must contain a 'main.dart' entry.");
     }
     DiagnosticCollector collector = new DiagnosticCollector();
 
-    bool oldBackendIsDart;
-    if (cachedCompiler != null) {
-      oldBackendIsDart = cachedCompiler.backend is DartBackend;
-    }
-    bool newBackendIsDart = template.options.contains('--output-type=dart');
-
     Compiler compiler = compilerFor(
         memorySourceFiles: example,
         diagnosticHandler: collector,
         options: [Flags.analyzeOnly,
                   Flags.enableExperimentalMirrors]..addAll(template.options),
-        cachedCompiler:
-             // TODO(johnniwinther): Remove this restriction when constant
-             // values can be computed directly from the expressions.
-             oldBackendIsDart == newBackendIsDart ? cachedCompiler : null);
+        cachedCompiler: cachedCompiler);
 
     return compiler.run(Uri.parse('memory:main.dart')).then((_) {
       Iterable<CollectedMessage> messages = collector.filterMessagesByKinds(
           [Diagnostic.ERROR,
            Diagnostic.WARNING,
            Diagnostic.HINT,
            Diagnostic.CRASH]);
 
       Expect.isFalse(messages.isEmpty, 'No messages in """$example"""');
 
@@ skipping 55 matching lines @@
                     kindsWithPendingClasses.contains(template));
 
       if (!pendingStuff) {
         // If there is pending stuff, or the compiler was cancelled, we
         // shouldn't reuse the compiler.
         cachedCompiler = compiler;
       }
     });
   }).then((_) => cachedCompiler);
 }