Report HintCode.MISSING_RETURN for async functions with return type that matters.

pkg/analyzer/lib/dart/element/type.dart

Index: pkg/analyzer/lib/dart/element/type.dart
diff --git a/pkg/analyzer/lib/dart/element/type.dart b/pkg/analyzer/lib/dart/element/type.dart
index 2ff00f0d4219bfaa9bffc68fe2eca5c6d4947318..fe0776559211f1859361c85caf551b82a1ac68ef 100644
--- a/pkg/analyzer/lib/dart/element/type.dart
+++ b/pkg/analyzer/lib/dart/element/type.dart
@@ -70,6 +70,12 @@ abstract class DartType implements ResolutionType {
   bool get isDartCoreFunction;
 
   /**
+   * Return `true` if this type represents the type 'Null' defined in the
+   * dart:core library.
+   */
+  bool get isDartCoreNull;
+
+  /**
    * Return `true` if this type represents the type 'dynamic'.
    */
   bool get isDynamic;