fix #26404, incorrect cast on async function return

pkg/analyzer/lib/src/task/strong/checker.dart

 // Copyright (c) 2015, 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.
 
 // TODO(jmesserly): this was ported from package:dev_compiler, and needs to be
 // refactored to fit into analyzer.
 library analyzer.src.task.strong.checker;
 
 import 'package:analyzer/analyzer.dart';
 import 'package:analyzer/dart/ast/ast.dart';
@@ skipping 702 matching lines @@
   void _checkReturnOrYield(Expression expression, AstNode node,
       {bool yieldStar: false}) {
     FunctionBody body = node.getAncestor((n) => n is FunctionBody);
     var type = _getExpectedReturnType(body, yieldStar: yieldStar);
     if (type == null) {
       // We have a type mismatch: the async/async*/sync* modifier does
       // not match the return or yield type.  We should have already gotten an
       // analyzer error in this case.
       return;
     }
-    InterfaceType futureType = typeProvider.futureType;
-    DartType actualType = expression?.staticType;
-    if (body.isAsynchronous &&
-        !body.isGenerator &&
-        actualType is InterfaceType &&
-        actualType.element == futureType.element) {
-      type = futureType.instantiate([type]);
-    }
     // TODO(vsm): Enforce void or dynamic (to void?) when expression is null.
     if (expression != null) checkAssignment(expression, type);
   }
 
   void _checkRuntimeTypeCheck(AstNode node, TypeName typeName) {
     var type = getType(typeName);
     if (!rules.isGroundType(type)) {
       _recordMessage(node, StrongModeCode.NON_GROUND_TYPE_CHECK_INFO, [type]);
     }
   }
@@ skipping 110 matching lines @@
     }
 
     var type = functionType.returnType;
 
     InterfaceType expectedType = null;
     if (body.isAsynchronous) {
       if (body.isGenerator) {
         // Stream<T> -> T
         expectedType = typeProvider.streamType;
       } else {
-        // Future<T> -> T
-        // TODO(vsm): Revisit with issue #228.
-        expectedType = typeProvider.futureType;
+        // Don't validate return type of async methods.
+        // They're handled by the runtime implementation.
+        return null;
       }
     } else {
       if (body.isGenerator) {
         // Iterable<T> -> T
         expectedType = typeProvider.iterableType;
       } else {
         // T -> T
         return type;
       }
     }
@@ skipping 501 matching lines @@
     var visited = new Set<InterfaceType>();
     do {
       visited.add(current);
       current.mixins.reversed.forEach(
           (m) => _checkIndividualOverridesFromClass(node, m, seen, true));
       _checkIndividualOverridesFromClass(node, current.superclass, seen, true);
       current = current.superclass;
     } while (!current.isObject && !visited.contains(current));
   }
 }