Add ClassDeclaration/ClassTypeAlias.isAbstract() and use them.

editor/tools/plugins/com.google.dart.engine/src/com/google/dart/engine/internal/builder/ElementBuilder.java

 /*
  * Copyright (c) 2012, the Dart project authors.
  * 
  * Licensed under the Eclipse Public License v1.0 (the "License"); you may not use this file except
  * in compliance with the License. You may obtain a copy of the License at
  * 
  * http://www.eclipse.org/legal/epl-v10.html
  * 
  * Unless required by applicable law or agreed to in writing, software distributed under the License
  * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
@@ skipping 205 matching lines @@
     interfaceType.setTypeArguments(typeArguments);
     element.setType(interfaceType);
 
     ConstructorElement[] constructors = holder.getConstructors();
     if (constructors.length == 0) {
       //
       // Create the default constructor.
       //
       constructors = createDefaultConstructors(interfaceType);
     }
-    element.setAbstract(node.getAbstractKeyword() != null);
+    element.setAbstract(node.isAbstract());
     element.setAccessors(holder.getAccessors());
     element.setConstructors(constructors);
     element.setFields(holder.getFields());
     element.setMethods(holder.getMethods());
     element.setTypeParameters(typeParameters);
     element.setValidMixin(isValidMixin);
 
     int functionTypeCount = functionTypesToFix.size();
     for (int i = 0; i < functionTypeCount; i++) {
       functionTypesToFix.get(i).setTypeArguments(typeArguments);
@@ skipping 723 matching lines @@
       ElementHolder previousHolder = currentHolder;
       currentHolder = holder;
       try {
         node.visitChildren(this);
       } finally {
         currentHolder = previousHolder;
       }
     }
   }
 }