Add a getter to TypeProvider for Future<Null>.

pkg/analyzer/lib/src/generated/error_verifier.dart

Index: pkg/analyzer/lib/src/generated/error_verifier.dart
diff --git a/pkg/analyzer/lib/src/generated/error_verifier.dart b/pkg/analyzer/lib/src/generated/error_verifier.dart
index 0ea52160c5b5ec2baa950aaa8edb5facab13e5ab..fdefc3b6cb1ee80fe1c0c5a745472f0aafcfa338 100644
--- a/pkg/analyzer/lib/src/generated/error_verifier.dart
+++ b/pkg/analyzer/lib/src/generated/error_verifier.dart
@@ -5778,8 +5778,7 @@ class ErrorVerifier extends RecursiveAstVisitor<Object> {
     // TODO(paulberry): do the right thing for generators.
     if (returnExpression == null) {
       if (_enclosingFunction.isAsynchronous) {
-        return _typeProvider.futureType.substitute4(
-            <DartType>[_typeProvider.nullType]);
+        return _typeProvider.futureNullType;
       } else {
         return VoidTypeImpl.instance;
       }