add corejs aliasing transformer and support for optional transformers

test/fixtures/transformation/optional-core-aliasing/es6-destructuring/actual.js

@@ -0,0 +1 @@
+var [a, [b], [c], d] = ["hello", [", ", "junk"], ["world"]];

test/fixtures/transformation/optional-core-aliasing/es6-destructuring/expected.js

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+"use strict";
+
+var _slicedToArray = function (arr, i) {
+  if (Array.isArray(arr)) {
+    return arr;
+  } else {
+    var _arr = [];
+    for (var _iterator = _core.$for.getIterator(arr), _step; !(_step = _iterator.next()).done;) {
+      _arr.push(_step.value);
+
+      if (i && _arr.length === i) break;
+    }
+
+    return _arr;
+  }
+};
+
+var _interopRequire = function (obj) {
+  return obj && (obj["default"] || obj);
+};
+
+var _core = _interopRequire(require("core-js/library"));
+
+var _ref = ["hello", [", ", "junk"], ["world"]];
+
+var _ref2 = _slicedToArray(_ref, 4);
+
+var a = _ref2[0];
+var _ref3 = _slicedToArray(_ref2[1], 1);
+
+var b = _ref3[0];
+var _ref4 = _slicedToArray(_ref2[2], 1);
+
+var c = _ref4[0];
+var d = _ref2[3];

test/fixtures/transformation/optional-core-aliasing/es6-for-of/actual.js

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+for (var i of arr) {
+
+}

test/fixtures/transformation/optional-core-aliasing/es6-for-of/expected.js

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+"use strict";
+
+var _interopRequire = function (obj) {
+  return obj && (obj["default"] || obj);
+};
+
+var _core = _interopRequire(require("core-js/library"));
+
+for (var _iterator = _core.$for.getIterator(arr), _step; !(_step = _iterator.next()).done;) {
+  var i = _step.value;
+}

test/fixtures/transformation/optional-core-aliasing/es6-spread/actual.js

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+var lyrics = ["head", "and", "toes", ...parts];

test/fixtures/transformation/optional-core-aliasing/es6-spread/expected.js

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+"use strict";
+
+var _toArray = function (arr) {
+  return Array.isArray(arr) ? arr : _core.Array.from(arr);
+};
+
+var _interopRequire = function (obj) {
+  return obj && (obj["default"] || obj);
+};
+
+var _core = _interopRequire(require("core-js/library"));
+
+var lyrics = ["head", "and", "toes"].concat(_toArray(parts));

test/fixtures/transformation/optional-core-aliasing/es7-array-comprehensions/actual.js

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+var arr = [for (i of nums) i * i];

test/fixtures/transformation/optional-core-aliasing/es7-array-comprehensions/expected.js

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+"use strict";
+
+var _toArray = function (arr) {
+  return Array.isArray(arr) ? arr : _core.Array.from(arr);
+};
+
+var _interopRequire = function (obj) {
+  return obj && (obj["default"] || obj);
+};
+
+var _core = _interopRequire(require("core-js/library"));
+
+var arr = _toArray(nums).map(function (i) {
+  return i * i;
+});

test/fixtures/transformation/optional-core-aliasing/es7-object-spread-rest/actual.js

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+var z = { ...x };

test/fixtures/transformation/optional-core-aliasing/es7-object-spread-rest/expected.js

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+"use strict";
+
+var _interopRequire = function (obj) {
+  return obj && (obj["default"] || obj);
+};
+
+var _core = _interopRequire(require("core-js/library"));
+
+var z = _core.Object.assign({}, x);

test/fixtures/transformation/optional-core-aliasing/options.json

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+{
+  "optional": ["coreAliasing"],
+  "experimental": true
+}