Make Invocation.positionalArguments and .namedArguments never be null.

sdk/lib/core/invocation.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.
 
 part of dart.core;
 
 /**
  * Representation of the invocation of a member on an object.
  *
  * This is the type of objects passed to [Object.noSuchMethod] when
  * an object doesn't support the member invocation that was attempted
  * on it.
  */
 abstract class Invocation {
   /** The name of the invoked member. */
   Symbol get memberName;
 
-  /** An unmodifiable view of the positional arguments of the call. */
+  /**
+   * An unmodifiable view of the positional arguments of the call.
+   *
+   * If the member is a getter, the positional arguments is empty.
+   */
   List get positionalArguments;
 
-  /** An unmodifiable view of the named arguments of the call. */
+  /**
+   * An unmodifiable view of the named arguments of the call.
+   *
+   * If the member is a getter, setter or operator, the named arguments
+   * is empty.
+   */
   Map<Symbol, dynamic> get namedArguments;
 
   /** Whether the invocation was a method call. */
   bool get isMethod;
 
   /**
    * Whether the invocation was a getter call.
-   * If so, both types of arguments will be null.
+   * If so, both types of arguments is empty.
    */
   bool get isGetter;
 
   /**
    * Whether the invocation was a setter call.
    *
-   * If so, [arguments] will have exactly one positonal argument,
-   * and namedArguments will be null.
+   * If so, [arguments] has exactly one positonal argument,
+   * and [namedArguments] is empty.
    */
   bool get isSetter;
 
   /** Whether the invocation was a getter or a setter call. */
   bool get isAccessor => isGetter || isSetter;
 }