Don't use _possibleConstructorReturn inside arrow functions (#6103)

Arrow functions can't be entrly skipped while traversing because this
references inside of them needs to be transformed, so I added a check
which prevents return statements inside arrow functions from being
saved for the transformation.

Fixes #5817 (regression)

packages/babel-plugin-transform-es2015-classes/test/fixtures/regression/5817/actual.js

@@ -0,0 +1,8 @@
+class A extends B {
+  constructor() {
+    super();
+
+    this.arrow1 = (x) => { return x; };
+    this.arrow2 = (x) => x;
+  }
+}

packages/babel-plugin-transform-es2015-classes/test/fixtures/regression/5817/exec.js

@@ -0,0 +1,16 @@
+// https://github.com/babel/babel/issues/5817
+
+class Parent {}
+
+class Table extends Parent {
+  constructor() {
+    super();
+    this.returnParam = (param) => {
+      return param;
+    }
+  }
+}
+
+const table = new Table();
+
+assert(table.returnParam(false) === false);

packages/babel-plugin-transform-es2015-classes/test/fixtures/regression/5817/expected.js

@@ -0,0 +1,24 @@
+"use strict";
+
+var A = function (_B) {
+  babelHelpers.inherits(A, _B);
+
+  function A() {
+    var _this;
+
+    babelHelpers.classCallCheck(this, A);
+    _this = babelHelpers.possibleConstructorReturn(this, (A.__proto__ || Object.getPrototypeOf(A)).call(this));
+
+    _this.arrow1 = function (x) {
+      return x;
+    };
+
+    _this.arrow2 = function (x) {
+      return x;
+    };
+
+    return _this;
+  }
+
+  return A;
+}(B);