fix #25487, infer block lambdas from return statements

pkg/analyzer/test/src/context/mock_sdk.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.test.src.context.mock_sdk;
 
 import 'package:analyzer/file_system/file_system.dart' as resource;
 import 'package:analyzer/file_system/memory_file_system.dart' as resource;
 import 'package:analyzer/src/context/cache.dart';
 import 'package:analyzer/src/context/context.dart';
@@ skipping 94 matching lines @@
 abstract class num implements Comparable<num> {
   bool operator <(num other);
   bool operator <=(num other);
   bool operator >(num other);
   bool operator >=(num other);
   num operator +(num other);
   num operator -(num other);
   num operator *(num other);
   num operator /(num other);
   int toInt();
+  double toDouble();
   num abs();
   int round();
 }
 abstract class int extends num {
   bool get isEven => false;
   int operator -();
   external static int parse(String source,
                             { int radix,
                               int onError(String source) });
 }
@@ skipping 252 matching lines @@
 
   @override
   AnalysisCache createCacheFromSourceFactory(SourceFactory factory) {
     if (factory == null) {
       return super.createCacheFromSourceFactory(factory);
     }
     return new AnalysisCache(
         <CachePartition>[AnalysisEngine.instance.partitionManager.forSdk(sdk)]);
   }
 }