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

pkg/analyzer/lib/task/model.dart

Index: pkg/analyzer/lib/task/model.dart
diff --git a/pkg/analyzer/lib/task/model.dart b/pkg/analyzer/lib/task/model.dart
index 58e095bca1f413af86902aeedde3aebb2030d9d3..8144f17ee1e130b21c5c7abf9dc405018a05826c 100644
--- a/pkg/analyzer/lib/task/model.dart
+++ b/pkg/analyzer/lib/task/model.dart
@@ -171,6 +171,17 @@ abstract class AnalysisTask {
   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.
    */