Add support for the "external" keyword to summaries.

pkg/analyzer/lib/src/summary/summarize_elements.dart

 // Copyright (c) 2015, 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 serialization.elements;
 
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/dart/element/type.dart';
 import 'package:analyzer/src/dart/element/type.dart';
 import 'package:analyzer/src/generated/resolver.dart';
@@ skipping 328 matching lines @@
       b.kind = UnlinkedExecutableKind.constructor;
       b.isConst = executableElement.isConst;
       b.isFactory = executableElement.isFactory;
     } else {
       b.kind = UnlinkedExecutableKind.functionOrMethod;
     }
     b.isAbstract = executableElement.isAbstract;
     b.isStatic = executableElement.isStatic &&
         executableElement.enclosingElement is ClassElement;
     b.hasImplicitReturnType = executableElement.hasImplicitReturnType;
+    b.isExternal = executableElement.isExternal;
     return b;
   }
 
   /**
    * Serialize the given [exportElement] into an [UnlinkedExport].
    */
   UnlinkedExportBuilder serializeExport(ExportElement exportElement) {
     UnlinkedExportBuilder b = new UnlinkedExportBuilder(ctx);
     b.uri = exportElement.uri;
     b.combinators = exportElement.combinators.map(serializeCombinator).toList();
@@ skipping 194 matching lines @@
     UnlinkedVariableBuilder b = new UnlinkedVariableBuilder(ctx);
     b.name = variable.name;
     b.type = serializeTypeRef(variable.type, variable);
     b.isStatic = variable.isStatic && variable.enclosingElement is ClassElement;
     b.isFinal = variable.isFinal;
     b.isConst = variable.isConst;
     b.hasImplicitType = variable.hasImplicitType;
     return b;
   }
 }