Fix warnings (#1606)

* Bump required compiler version to 0.5.2.

* Fix shadowed variable warning in ERC20Migrator.

* Rename Counter to Counters.

* Add dummy state variable to SafeERC20Helper.

* Update changelog entry.

* Fix CountersImpl name.

* Improve changelog entry.

test/drafts/Counters.test.js

@@ -0,0 +1,37 @@
+const { BN } = require('openzeppelin-test-helpers');
+
+const CountersImpl = artifacts.require('CountersImpl');
+
+const EXPECTED = [new BN(1), new BN(2), new BN(3), new BN(4)];
+const KEY1 = web3.utils.sha3('key1');
+const KEY2 = web3.utils.sha3('key2');
+
+contract('Counters', function ([_, owner]) {
+  beforeEach(async function () {
+    this.mock = await CountersImpl.new({ from: owner });
+  });
+
+  context('custom key', async function () {
+    it('should return expected values', async function () {
+      for (const expectedId of EXPECTED) {
+        await this.mock.doThing(KEY1, { from: owner });
+        const actualId = await this.mock.theId();
+        actualId.should.be.bignumber.equal(expectedId);
+      }
+    });
+  });
+
+  context('parallel keys', async function () {
+    it('should return expected values for each counter', async function () {
+      for (const expectedId of EXPECTED) {
+        await this.mock.doThing(KEY1, { from: owner });
+        let actualId = await this.mock.theId();
+        actualId.should.be.bignumber.equal(expectedId);
+
+        await this.mock.doThing(KEY2, { from: owner });
+        actualId = await this.mock.theId();
+        actualId.should.be.bignumber.equal(expectedId);
+      }
+    });
+  });
+});