add support for flow type casts and module types

lib/6to5/generation/generators/flow.js

@@ -30,6 +30,7 @@ exports.ObjectTypeIndexer =
 exports.ObjectTypeProperty =
 exports.QualifiedTypeIdentifier =
 exports.UnionTypeAnnotation =
+exports.TypeCastExpression =
 exports.VoidTypeAnnotation = function () {
   // todo: implement these once we have a `--keep-types` option
 };

lib/6to5/transformation/transformers/index.js

@@ -12,6 +12,7 @@ module.exports = {
   "playground.objectGetterMemoization":    require("./playground/object-getter-memoization"),
 
   reactCompat:                             require("./other/react-compat"),
+  flow:                                    require("./other/flow"),
   react:                                   require("./other/react"),
 
   _modules:                                require("./internal/modules"),

lib/6to5/transformation/transformers/other/flow.js

@@ -0,0 +1,13 @@
+var t = require("../../../types");
+
+exports.TypeCastExpression = function (node) {
+  return node.expression;
+};
+
+exports.ImportDeclaration = function (node) {
+  if (node.type) this.remove();
+};
+
+exports.ExportDeclaration = function (node) {
+  if (t.isTypeAlias(node.declaration)) this.remove();
+};

lib/6to5/types/visitor-keys.json

@@ -92,6 +92,7 @@
   "TypeofTypeAnnotation":        [],
   "TypeAlias":                   [],
   "TypeAnnotation":              [],
+  "TypeCastExpression":          ["expression"],
   "TypeParameterDeclaration":    [],
   "TypeParameterInstantiation":  [],
   "ObjectTypeAnnotation":        [],