Tolerate SDKs that do not include dart:async (issue 25520)

pkg/analyzer/lib/task/model.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 analyzer.task.model;
 
 import 'dart:collection';
 import 'dart:developer';
 
 import 'package:analyzer/src/generated/engine.dart' hide AnalysisTask;
@@ skipping 153 matching lines @@
 
   /**
    * Indicates whether the task is capable of handling dependency cycles.  A
    * task that overrides this getter to return `true` must be prepared for the
    * possibility that it will be invoked with a non-`null` value of
    * [dependencyCycle], and with not all of its inputs computed.
    */
   bool get handlesDependencyCycles => false;
 
   /**
+   * Return the value of the input with the given [name], or `null` if the input
+   * value is not defined.
+   */
+  Object getOptionalInput(String name) {
+    if (inputs == null || !inputs.containsKey(name)) {
+      return null;
+    }
+    return inputs[name];
+  }
+
+  /**
    * Return the value of the input with the given [name]. Throw an exception if
    * the input value is not defined.
    */
   Object getRequiredInput(String name) {
     if (inputs == null || !inputs.containsKey(name)) {
       throw new AnalysisException("Could not $description: missing $name");
     }
     return inputs[name];
   }
 
@@ skipping 569 matching lines @@
   /**
    * A work should be done, but without any special urgency.
    */
   NORMAL,
 
   /**
    * Nothing to do.
    */
   NONE
 }