```pkg/collection: fix version

updated unittest max version
removed unused import

Review URL: https://codereview.chromium.org//182153002

```
2 files changed
tree: 21dc8900b3df872aefa5f9454b90c5b889f7dab9

Helper libraries for working with collections.

The `collection` package contains a number of separate libraries with utility functions and classes that makes working with collections easier.

## Using

The `collection` package can be imported as separate libraries, or in totality:

```import 'package:collection/algorithms.dart';
import 'package:collection/equality.dart';
import 'package:collection/iterable_zip.dart';
import 'package:collection/priority_queue.dart';
import 'package:collection/wrappers.dart';
```

or

```import 'package:collection/collection.dart';
```

## Algorithms

The algorithms library contains functions that operate on lists.

It contains ways to shuffle a `List`, do binary search on a sorted `List`, and various sorting algorithms.

## Equality

The equality library gives a way to specify equality of elements and collections.

Collections in Dart have no inherent equality. Two sets are not equal, even if they contain exactly the same objects as elements.

The equality library provides a way to say define such an equality. In this case, for example, `const SetEquality(const IdentityEquality())` is an equality that considers two sets equal exactly if they contain identical elements.

The library provides ways to define equalities on `Iterable`s, `List`s, `Set`s, and `Map`s, as well as combinations of these, such as:

```const MapEquality(const IdentityEquality(), const ListEquality());
```

This equality considers maps equal if they have identical keys, and the corresponding values are lists with equal (`operator==`) values.

## Iterable Zip

Utilities for “zipping” a list of iterables into an iterable of lists.

## Priority Queue

An interface and implemention of a priority queue.

## Wrappers

The wrappers library contains classes that “wrap” a collection.

A wrapper class contains an object of the same type, and it forwards all methods to the wrapped object.

Wrapper classes can be used in various ways, for example to restrict the type of an object to that of a supertype, or to change the behavior of selected functions on an existing object.