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

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

Index: pkg/analyzer/lib/src/dart/element/type.dart
diff --git a/pkg/analyzer/lib/src/dart/element/type.dart b/pkg/analyzer/lib/src/dart/element/type.dart
index 968c18fa1a85581e1382e38008800b7220269c93..dab72dcd6a70cc0dc0a8eb023013f18297cc3e36 100644
--- a/pkg/analyzer/lib/src/dart/element/type.dart
+++ b/pkg/analyzer/lib/src/dart/element/type.dart
@@ -1396,6 +1396,15 @@ class InterfaceTypeImpl extends TypeImpl implements InterfaceType {
   }
 
   @override
+  bool get isDartCoreNull {
+    ClassElement element = this.element;
+    if (element == null) {
+      return false;
+    }
+    return element.name == "Null" && element.library.isDartCore;
+  }
+
+  @override
   bool get isObject => element.supertype == null;
 
   @override
@@ -2371,6 +2380,9 @@ abstract class TypeImpl implements DartType {
   bool get isDartCoreFunction => false;
 
   @override
+  bool get isDartCoreNull => false;
+
+  @override
   bool get isDynamic => false;
 
   @override
@@ -2551,10 +2563,10 @@ class TypeParameterTypeImpl extends TypeImpl implements TypeParameterType {
       : super(element, element.name);
 
   @override
-  ElementLocation get definition => element.location;
+  DartType get bound => element.bound ?? DynamicTypeImpl.instance;
 
   @override
-  DartType get bound => element.bound ?? DynamicTypeImpl.instance;
+  ElementLocation get definition => element.location;
 
   @override
   TypeParameterElement get element => super.element as TypeParameterElement;