fixes #27586, prefer context type in generic inference

tests/corelib_strong/set_test.dart

 // Copyright (c) 2011, 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 set_test;
 
 
 import 'package:expect/expect.dart';
 import "dart:collection";
 
@@ skipping 410 matching lines @@
 
   Iterable<num> numIter = numList.where((x) => true);
   Set<int> set3 = setFrom(numIter);
   Expect.listEquals(numList, set3.toList()..sort());
 
   Set<int> set4 = setFrom(new Iterable.generate(0));
   Expect.isTrue(set4.isEmpty);
 }
 
 void testCESetFrom(setFrom) {
-  List<Object> ceList = [new CE(2), new CE(3), new CE(5),
-                         new CE(7), new CE(11), new CE(13)];
+  var ceList = [new CE(2), new CE(3), new CE(5),
+                new CE(7), new CE(11), new CE(13)];
 
   Set<CE> set1 = setFrom(ceList);
   Expect.listEquals(ceList, set1.toList()..sort());
 
   Set<CE> ceSet = ceList.toSet();
   Set<CE> set2 = setFrom(ceSet);
   Expect.listEquals(ceList, set2.toList()..sort());
 
   Iterable<CE> ceIter = ceList.where((x) => true);
   Set<CE> set3 = setFrom(ceIter);
@@ skipping 76 matching lines @@
 
   testCESetFrom((x) => new SplayTreeSet<CE>.from(x,
                                                  customCompare(20), validKey));
   testCESetFrom((x) => new SplayTreeSet<CE>.from(x, identityCompare));
 
   testASetFrom((x) => new Set<A>.from(x));
   testASetFrom((x) => new HashSet<A>.from(x));
   testASetFrom((x) => new LinkedHashSet<A>.from(x));
   testASetFrom((x) => new SplayTreeSet<A>.from(x, identityCompare));
 }