Check that non-abstract classes implement all methods.

dart/sdk/lib/_internal/compiler/implementation/elements/modelx.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 elements.modelx;
 
 import 'elements.dart';
 import '../../compiler.dart' as api;
 import '../tree/tree.dart';
 import '../util/util.dart';
@@ skipping 1294 matching lines @@
           setter.modifiers.nodes,
           setter.modifiers.flags | Modifiers.FLAG_ABSTRACT);
     }
   }
 
   bool isInstanceMember() {
     return isMember() && !modifiers.isStatic();
   }
 
   accept(ElementVisitor visitor) => visitor.visitAbstractFieldElement(this);
+
+  bool get isAbstract {
+    return getter != null && getter.isAbstract
+        || setter != null && setter.isAbstract;
+  }
 }
 
 // TODO(johnniwinther): [FunctionSignature] should be merged with
 // [FunctionType].
 class FunctionSignatureX implements FunctionSignature {
   final Link<Element> requiredParameters;
   final Link<Element> optionalParameters;
   final DartType returnType;
   final int requiredParameterCount;
   final int optionalParameterCount;
@@ skipping 1109 matching lines @@
 
   MetadataAnnotation ensureResolved(Compiler compiler) {
     if (resolutionState == STATE_NOT_STARTED) {
       compiler.resolver.resolveMetadataAnnotation(this);
     }
     return this;
   }
 
   String toString() => 'MetadataAnnotation($value, $resolutionState)';
 }