fixes #25482, flatten Futures in strong mode so Future.then works

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

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library analyzer.src.generated.error_verifier;
 
 import 'dart:collection';
 import "dart:math" as math;
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/visitor.dart';
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/dart/element/type.dart';
 import 'package:analyzer/dart/element/visitor.dart';
 import 'package:analyzer/src/dart/element/element.dart';
 import 'package:analyzer/src/dart/element/member.dart';
 import 'package:analyzer/src/dart/element/type.dart';
 import 'package:analyzer/src/generated/constant.dart';
 import 'package:analyzer/src/generated/element_resolver.dart';
 import 'package:analyzer/src/generated/error.dart';
 import 'package:analyzer/src/generated/java_engine.dart';
 import 'package:analyzer/src/generated/parser.dart'
     show Parser, ParserErrorCode;
 import 'package:analyzer/src/generated/resolver.dart';
 import 'package:analyzer/src/generated/scanner.dart' as sc;
 import 'package:analyzer/src/generated/sdk.dart' show DartSdk, SdkLibrary;
-import 'package:analyzer/src/generated/static_type_analyzer.dart';
 import 'package:analyzer/src/generated/utilities_dart.dart';
 
 /**
  * A visitor used to traverse an AST structure looking for additional errors and
  * warnings not covered by the parser and resolver.
  */
 class ErrorVerifier extends RecursiveAstVisitor<Object> {
   /**
    * Static final string with value `"getter "` used in the construction of the
    * [StaticWarningCode.NON_ABSTRACT_CLASS_INHERITS_ABSTRACT_MEMBER_ONE], and
@@ skipping 5727 matching lines @@
     assert(!_inGenerator);
     if (returnExpression == null) {
       if (_enclosingFunction.isAsynchronous) {
         return _typeProvider.futureNullType;
       } else {
         return VoidTypeImpl.instance;
       }
     }
     DartType staticReturnType = getStaticType(returnExpression);
     if (staticReturnType != null && _enclosingFunction.isAsynchronous) {
-      return _typeProvider.futureType.substitute4(<DartType>[
-        StaticTypeAnalyzer.flattenFutures(_typeProvider, staticReturnType)
-      ]);
+      return _typeProvider.futureType.substitute4(
+          <DartType>[staticReturnType.flattenFutures(_typeSystem)]);
     }
     return staticReturnType;
   }
 
   /**
    * Return the error code that should be used when the given class [element]
    * references itself directly.
    */
   ErrorCode _getBaseCaseErrorCode(ClassElement element) {
     InterfaceType supertype = element.supertype;
@@ skipping 417 matching lines @@
     toCheck.add(type.element);
     // type arguments
     if (type is InterfaceType) {
       InterfaceType interfaceType = type;
       for (DartType typeArgument in interfaceType.typeArguments) {
         _addTypeToCheck(typeArgument);
       }
     }
   }
 }