Magento2 Fakes implementation built step by step to support our tests. (Context: https://mobile.twitter.com/fschmengler/status/859473111984590848)

FakeAddressRepository.php

<?php

namespace Terrang\Fluxs\Test\Fake\Repository;

use Magento\Framework\Exception\NoSuchEntityException;

class FakeAddressRepository implements \Magento\Customer\Api\AddressRepositoryInterface
{
    private $db = [];

    public function save(\Magento\Customer\Api\Data\AddressInterface $address)
    {
        $this->db[$address->getId()] = $address;
    }

    public function getById($addressId)
    {
        if (!array_key_exists($addressId, $this->db)) {
            throw NoSuchEntityException::singleField('addressId', $addressId);
        }
        return $this->db[$addressId];
    }

    public function getList(\Magento\Framework\Api\SearchCriteriaInterface $searchCriteria)
    {
        // TODO: Implement search filters
        return $this->db;
    }

    public function delete(\Magento\Customer\Api\Data\AddressInterface $address)
    {
        return $this->deleteById($address->getId());
    }

    public function deleteById($addressId)
    {
        $found = false;
        if (array_key_exists($addressId, $this->db)) {
            $found = true;
            unset($this->db[$addressId]);
        }
        return $found;
    }
}

FakeCollection.php

<?php

namespace Terrang\Fluxs\Test\Fake;

class FakeCollection extends FakeDb
{
    public function __construct() {
        parent::__construct();
    }

    public function getTable($table) {}
    public function setEntity($entity) {}
    public function getEntity() {}
    public function getResource() {}
    public function setObject($object = null) {
        return $this;
    }
    public function addItem(\Magento\Framework\DataObject $object) {
        return parent::_addItem($object);
    }
    public function getAttribute($attributeCode) {}
    public function addAttributeToFilter($attribute, $condition = null, $joinType = 'inner') {
        $this->addFieldToFilter($attribute, $condition);
        return $this;
    }
    public function addFieldToFilter($attribute, $condition = null) {
        $this->_filters[] = compact('attribute', 'condition');
        return $this;
    }
    public function addAttributeToSort($attribute, $dir = self::SORT_ORDER_ASC) {
        return $this;
    }
    public function addAttributeToSelect($attribute, $joinType = false) {
        return $this;
    }
    public function addEntityTypeToSelect($entityType, $prefix) {
        return $this;
    }
    public function addStaticField($field) {
        return $this;
    }
    public function addExpressionAttributeToSelect($alias, $expression, $attribute) {
        return $this;
    }
    public function groupByAttribute($attribute) {
        return $this;
    }
    public function joinAttribute($alias, $attribute, $bind, $filter = null, $joinType = 'inner', $storeId = null) {
        return $this;
    }
    public function joinField($alias, $table, $field, $bind, $cond = null, $joinType = 'inner') {
        return $this;
    }
    public function joinTable($table, $bind, $fields = null, $cond = null, $joinType = 'inner') {
        return $this;
    }
    public function removeAttributeToSelect($attribute = null) {
        return $this;
    }
    public function setPage($pageNum, $pageSize) {
        $this->_curPage = $pageNum;
        $this->_pageSize = $pageSize;
        return $this;
    }
    public function load($printQuery = false, $logQuery = false) {
        parent::load();
        if ($this->_pageSize) {
            $startIndex = $this->_pageSize * ($this->_curPage - 1);
            $this->_items = array_slice($this->_items, $startIndex, $this->_pageSize);
        }
        return $this;
    }
    public function getAllIds($limit = null, $offset = null) {}
    public function getAllIdsSql() {}
    public function save() {
        return $this;
    }
    public function delete() {
        return $this;
    }
    public function exportToArray() {
        return $this->_items;
    }
    public function getRowIdFieldName() {}
    public function getIdFieldName() {}
    public function setRowIdFieldName($fieldName) {
        return $this;
    }
    public function _loadEntities($printQuery = false, $logQuery = false) {
        return $this;
    }
    public function _loadAttributes($printQuery = false, $logQuery = false) {
        return $this;
    }
    public function setOrder($attribute, $dir = self::SORT_ORDER_ASC) {
        return $this;
    }
    public function toArray($arrAttributes = []) {
        return parent::toArray($arrAttributes);
    }
    public function getLoadedIds() {}
    public function clear() {
        return parent::clear();
    }
    public function removeAllItems() {
        return $this;
    }
    public function removeItemByKey($key) {
        return $this;
    }
    public function getMainTable() {}
    public function addFieldToSelect($field, $alias = null) {
        return $this;
    }
    public function removeFieldFromSelect($field) {
        return $this;
    }
    public function removeAllFieldsFromSelect() {
        return $this;
    }
}

FakeDb.php

<?php

namespace Terrang\Fluxs\Test\Fake;

use Magento\Framework\Api\ExtensionAttribute\JoinDataInterface;
use Magento\Framework\Api\ExtensionAttribute\JoinProcessorInterface;

/**
 * Overrides all public method of the AbstractDB
 * to provide a simple in memory implementation
 * for use in tests
 *
 * @package Terrang\Fluxs\Test\Fake
 */
class FakeDb extends \Magento\Framework\Data\Collection\AbstractDb
{
    /**
     * In memory database content
     * Almost the same than _items but in case of pagination items will
     * be a slice of it
     *
     * @var array
     */
    protected $__db;

    public function __construct() {
        $this->populateWith([]);
    }

    public function populateWith(array $items)
    {
        $this->__db = $items;
        $this->_items = $items;
    }
    public function getResource() {}
    public function addBindParam($name, $value) {}
    public function getIdFieldName() {}
    public function setConnection(\Magento\Framework\DB\Adapter\AdapterInterface $conn) {}
    public function getSelect() {}
    public function getConnection() {}
    public function getSize() {
        return count($this->_items);
    }
    public function getSelectCountSql() {}
    public function getSelectSql($stringMode = false) {}
    public function setOrder($field, $direction = self::SORT_ORDER_DESC) {}
    public function addOrder($field, $direction = self::SORT_ORDER_DESC) {}
    public function unshiftOrder($field, $direction = self::SORT_ORDER_DESC) {}
    public function addFieldToFilter($field, $condition = null) {}
    public function distinct($flag) {}
    public function load($printQuery = false, $logQuery = false) {
        $this->_items = array_reduce($this->_filters, [$this, 'applyFilter'], $this->__db);
        return $this;
    }
    public function loadWithFilter($printQuery = false, $logQuery = false) {}
    public function fetchItem() {}
    public function getData() {}
    public function resetData() {}
    public function loadData($printQuery = false, $logQuery = false) {}
    public function printLogQuery($printQuery = false, $logQuery = false, $sql = null) {}
    public function addFilterToMap($filter, $alias, $group = 'fields') {}
    public function __clone() {}
    public function joinExtensionAttribute(JoinDataInterface $join, JoinProcessorInterface $extensionAttributesJoinProcessor) {}
    public function getItemObjectClass() {}
    public function __sleep() {}
    public function __wakeup() {}

    /** INTERNAL IMPLEMENTATIONS (private API) */
    private function applyFilter(array $data, array $filter)
    {
        $conditionMatcher = $this->makeConditionMatcher($filter['condition']);
        $attribute = $filter['attribute'];
        return array_values(array_filter($data, function (\Magento\Framework\DataObject $row) use ($attribute, $conditionMatcher) {
            return $conditionMatcher($row->getData($attribute));
        }));
    }

    private function makeConditionMatcher($condition)
    {
        $trueMatcher = function ($value) {
            return true;
        };
        $inMatcher = function ($allowedValues) {
            return function($value) use ($allowedValues) {
                return in_array($value, $allowedValues);
            };
        };

        if (!empty($condition['in'])) {
            return $inMatcher($condition['in']);
        }
        return $trueMatcher;
    }
}