Make corelib_strong tests strong-mode compliant

tests/corelib_strong/iterable_skip_test.dart

 // Copyright (c) 2012, 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.
 
 import "package:expect/expect.dart";
 
 main() {
   List<int> list1 = <int>[1, 2, 3];
   List<int> list2 = const <int>[4, 5];
   List<String> list3 = <String>[];
@@ skipping 197 matching lines @@
   Expect.isNull(it.current);
   Expect.isFalse(it.moveNext());
   Expect.isNull(it.current);
 
   skip4 = set1.skip(4);
   it = skip4.iterator;
   Expect.isNull(it.current);
   Expect.isFalse(it.moveNext());
   Expect.isNull(it.current);
 
-  skip0 = set2.skip(0);
-  it = skip0.iterator;
-  Expect.isNull(it.current);
-  Expect.isFalse(it.moveNext());
-  Expect.isNull(it.current);
+  var dynamicSkip0 = set2.skip(0);

[Leaf, 2017/02/08 02:03:49]

Alternatively, could make the set2 declaration be Set<int>, since it doesn't
look like the type is material.  I'm fine either way.

+  var dynamicIt = dynamicSkip0.iterator;
+  Expect.isNull(dynamicIt.current);
+  Expect.isFalse(dynamicIt.moveNext());
+  Expect.isNull(dynamicIt.current);
 
-  skip1 = set2.skip(1);
-  it = skip1.iterator;
-  Expect.isNull(it.current);
-  Expect.isFalse(it.moveNext());
-  Expect.isNull(it.current);
+  var dynamicSkip1 = set2.skip(1);
+  dynamicIt = dynamicSkip1.iterator;
+  Expect.isNull(dynamicIt.current);
+  Expect.isFalse(dynamicIt.moveNext());
+  Expect.isNull(dynamicIt.current);
 
   testSkipTake(Iterable input, int skip, int take) {
     List expected = [];
     Iterator iter = input.iterator;
     for (int i = 0; i < skip; i++) iter.moveNext();
     for (int i = 0; i < take; i++) {
       if (!iter.moveNext()) break;
       expected.add(iter.current);
     }
     Expect.listEquals(expected, input.skip(skip).take(take).toList());
@@ skipping 11 matching lines @@
     testSkipTake(collection, 5, 5);
     testSkipTake(collection, 5, 10);
     testSkipTake(collection, 5, 20);
     testSkipTake(collection, 15, 0);
     testSkipTake(collection, 15, 5);
     testSkipTake(collection, 20, 0);
     testSkipTake(collection, 20, 5);
     Expect.throws(() => longList.skip(-1), (e) => e is RangeError);
   }
 }