Make isEmpty a getter.

tests/compiler/dart2js/mock_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.
 
 library mock_compiler;
 
 import 'dart:uri';
 
 import '../../../lib/compiler/compiler.dart' as api;
 import '../../../lib/compiler/implementation/dart2jslib.dart' hide TreeElementMapping;
@@ skipping 124 matching lines @@
       errors.add(diagnostic);
     } else {
       warnings.add(diagnostic);
     }
   }
 
   void reportDiagnostic(SourceSpan span, String message, var kind) {
     print(message);
   }
 
-  bool get compilationFailed => !errors.isEmpty();
+  bool get compilationFailed => !errors.isEmpty;
 
   void clearWarnings() {
     warnings = [];
   }
 
   void clearErrors() {
     errors = [];
   }
 
   TreeElementMapping resolveStatement(String text) {
@@ skipping 75 matching lines @@
     });
   }
 }
 
 LibraryElement mockLibrary(Compiler compiler, String source) {
   Uri uri = new Uri.fromComponents(scheme: "source");
   var library = new LibraryElement(new Script(uri, new MockFile(source)));
   importLibrary(library, compiler.coreLibrary, compiler);
   return library;
 }