Issue 25623. When adding the 'async' modifier, change the return type.

pkg/analysis_server/lib/src/services/correction/fix_internal.dart

Index: pkg/analysis_server/lib/src/services/correction/fix_internal.dart
diff --git a/pkg/analysis_server/lib/src/services/correction/fix_internal.dart b/pkg/analysis_server/lib/src/services/correction/fix_internal.dart
index e223f815dab08891a41125cfcca482448ea17c56..c04bd7569d6887b913c1f023c83ff1687dce53f7 100644
--- a/pkg/analysis_server/lib/src/services/correction/fix_internal.dart
+++ b/pkg/analysis_server/lib/src/services/correction/fix_internal.dart
@@ -390,6 +390,7 @@ class FixProcessor {
       FunctionBody body = node.getAncestor((n) => n is FunctionBody);
       if (body != null && body.keyword == null) {
         _addReplaceEdit(rf.rangeStartLength(body, 0), 'async ');
+        _replaceReturnTypeWithFuture(body);
         _addFix(DartFixKind.ADD_ASYNC, []);
         return true;
       }
@@ -401,6 +402,7 @@ class FixProcessor {
     FunctionBody body = node.getAncestor((n) => n is FunctionBody);
     if (body != null && body.keyword == null) {
       _addReplaceEdit(rf.rangeStartLength(body, 0), 'async ');
+      _replaceReturnTypeWithFuture(body);
       _addFix(DartFixKind.ADD_ASYNC, []);
     }
   }
@@ -1357,19 +1359,9 @@ class FixProcessor {
   }
 
   void _addFix_illegalAsyncReturnType() {
-    InterfaceType futureType = context.typeProvider.futureType;
-    String futureTypeCode = utils.getTypeSource(futureType, librariesToImport);
     // prepare the existing type
     TypeName typeName = node.getAncestor((n) => n is TypeName);
-    String nodeCode = utils.getNodeText(typeName);
-    // wrap the existing type with Future
-    String returnTypeCode;
-    if (nodeCode == 'void') {
-      returnTypeCode = futureTypeCode;
-    } else {
-      returnTypeCode = '$futureTypeCode<$nodeCode>';
-    }
-    _addReplaceEdit(rf.rangeNode(typeName), returnTypeCode);
+    _replaceTypeWithFuture(typeName);
     // add proposal
     _addFix(DartFixKind.REPLACE_RETURN_TYPE_FUTURE, []);
   }
@@ -2787,6 +2779,40 @@ class FixProcessor {
     }
   }
 
+  void _replaceReturnTypeWithFuture(AstNode node) {
+    for (; node != null; node = node.parent) {
+      if (node is FunctionDeclaration) {
+        _replaceTypeWithFuture(node.returnType);
+        return;
+      } else if (node is MethodDeclaration) {
+        _replaceTypeWithFuture(node.returnType);
+        return;
+      }
+    }
+  }
+
+  void _replaceTypeWithFuture(TypeName typeName) {
+    InterfaceType futureType = context.typeProvider.futureType;
+    // validate the type
+    DartType type = typeName?.type;
+    if (type == null ||
+        type.isDynamic ||
+        type is InterfaceType && type.element == futureType.element) {
+      return;
+    }
+    // prepare code for the types
+    String futureTypeCode = utils.getTypeSource(futureType, librariesToImport);
+    String nodeCode = utils.getNodeText(typeName);
+    // wrap the existing type with Future
+    String returnTypeCode;
+    if (nodeCode == 'void') {
+      returnTypeCode = futureTypeCode;
+    } else {
+      returnTypeCode = '$futureTypeCode<$nodeCode>';
+    }
+    _addReplaceEdit(rf.rangeNode(typeName), returnTypeCode);
+  }
+
   void _updateFinderWithClassMembers(
       _ClosestElementFinder finder, ClassElement clazz) {
     if (clazz != null) {