Add support for the "external" keyword to summaries.

pkg/analyzer/tool/summary/idl.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.
 
 /**
  * This file is an "idl" style description of the summary format.  It is not
  * executed directly; instead it is parsed and transformed into code that
  * implements the summary format.
  *
  * The code generation process introduces the following non-typical semantics:
@@ skipping 333 matching lines @@
   /**
    * Indicates whether the executable is declared using the `factory` keyword.
    */
   bool isFactory;
 
   /**
    * Indicates whether the executable lacks an explicit return type
    * declaration.  False for constructors and setters.
    */
   bool hasImplicitReturnType;
+
+  /**
+   * Indicates whether the executable is declared using the `external` keyword.
+   */
+  bool isExternal;
 }
 
 /**
  * Enum used to indicate the kind of an executable.
  */
 enum UnlinkedExecutableKind {
   /**
    * Executable is a function or method.
    */
   functionOrMethod,
@@ skipping 339 matching lines @@
   /**
    * Indicates whether the variable is declared using the `const` keyword.
    */
   bool isConst;
 
   /**
    * Indicates whether this variable lacks an explicit type declaration.
    */
   bool hasImplicitType;
 }