Allow return of Future in async function

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/src/generated/ast.dart';
@@ skipping 595 matching lines @@
   void _checkReturnOrYield(Expression expression, AstNode node,
       {bool yieldStar: false}) {
     var body = node.getAncestor((n) => n is FunctionBody);
     var type = rules.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 = rules.provider.futureType;
+    DartType actualType = expression.staticType;
+    if (body.isAsynchronous &&
+        !body.isGenerator &&
+        actualType is InterfaceType &&
+        actualType.element == futureType.element) {
+      type = futureType.substitute4([type]);
+    }
     // TODO(vsm): Enforce void or dynamic (to void?) when expression is null.
     if (expression != null) checkAssignment(expression, type);
   }
 
   @override
   void visitExpressionFunctionBody(ExpressionFunctionBody node) {
     _checkReturnOrYield(node.expression, node);
     node.visitChildren(this);
   }
 
@@ skipping 416 matching lines @@
       }
     } catch (e) {
       // TODO(sigmund): remove this try-catch block (see issue #48).
     }
     if (baseMethod == null || baseMethod.isStatic) return null;
     return baseMethod.type;
   }
   ;
   return f;
 }