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 18 matching lines @@
   factory Future.value([T value]) => null;
 
   static Future<List/*<T>*/> wait/*<T>*/(
       Iterable<Future/*<T>*/> futures) => null;
   Future/*<R>*/ then/*<R>*/(/*=R*/ onValue(T value)) => null;
 }
 ''',
     const <String, String>{
       '/lib/async/stream.dart': r'''
 part of dart.async;
-class Stream<T> {}
+class Stream<T> {
+  Future<T> get first;
+}
 abstract class StreamTransformer<S, T> {}
 '''
     });
 
 const _MockSdkLibrary _LIB_COLLECTION = const _MockSdkLibrary(
     'dart:collection',
     '/lib/collection/collection.dart',
     '''
 library dart.collection;
 
@@ skipping 55 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) });
 }
 class double extends num {}
 class DateTime extends Object {}
 class Null extends Object {}
 
 class Deprecated extends Object {
   final String expires;
   const Deprecated(this.expires);
 }
 const Object deprecated = const Deprecated("next release");
 
 class Iterator<E> {
   bool moveNext();
   E get current;
 }
 
 abstract class Iterable<E> {
   Iterator<E> get iterator;
   bool get isEmpty;
+  E get first;
 
   Iterable/*<R>*/ map/*<R>*/(/*=R*/ f(E e));
 
   /*=R*/ fold/*<R>*/(/*=R*/ initialValue,
       /*=R*/ combine(/*=R*/ previousValue, E element));
 }
 
 abstract class List<E> implements Iterable<E> {
   void add(E value);
   E operator [](int index);
@@ skipping 224 matching lines @@
 
   @override
   AnalysisCache createCacheFromSourceFactory(SourceFactory factory) {
     if (factory == null) {
       return super.createCacheFromSourceFactory(factory);
     }
     return new AnalysisCache(
         <CachePartition>[AnalysisEngine.instance.partitionManager.forSdk(sdk)]);
   }
 }