Migration: add missing type annotation

Change-Id: Ie219297a4ffe203e0b15157471a2a59f1b1029a4
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/105960
Reviewed-by: Dan Rubel <danrubel@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
diff --git a/pkg/nnbd_migration/lib/src/node_builder.dart b/pkg/nnbd_migration/lib/src/node_builder.dart
index 30a778d..088f201 100644
--- a/pkg/nnbd_migration/lib/src/node_builder.dart
+++ b/pkg/nnbd_migration/lib/src/node_builder.dart
@@ -257,7 +257,7 @@
       FormalParameterList parameters,
       FunctionBody body,
       AstNode enclosingNode) {
-    var decoratedReturnType;
+    DecoratedType decoratedReturnType;
     if (returnType == null && declaredElement is ConstructorElement) {
       // Constructors have no explicit return type annotation, so use the
       // implicit return type.