[mdx-loader]: Better default export detection

This commit copies a bunch of code from @mdx-js/mdx. We should put
some effort in to merge the changes upstream. In the meantime I'm
publishing this as 0.0.14 to fix some user's problems.

fixes gatsbyjs#75

packages/gatsby-plugin-mdx/mdx-loader.js

@@ -1,11 +1,81 @@
 const _ = require("lodash");
 const { getOptions } = require("loader-utils");
 const grayMatter = require("gray-matter");
+const unified = require("unified");
+const {
+  isImport,
+  isExport,
+  isExportDefault,
+  BLOCKS_REGEX,
+  EMPTY_NEWLINE,
+  ...props
+} = require("@mdx-js/mdx/util");
+
+const toMDAST = require("remark-parse");
+const squeeze = require("remark-squeeze-paragraphs");
 
 const mdx = require("./utils/mdx");
 const debug = require("debug")("gatsby-mdx:mdx-loader");
 
-const hasDefaultExport = str => /\nexport default/.test(str);
+const DEFAULT_OPTIONS = {
+  footnotes: true,
+  mdPlugins: [],
+  hastPlugins: [],
+  compilers: [],
+  blocks: [BLOCKS_REGEX]
+};
+
+/**
+ * TODO: Find a way to PR all of this code that was lifted
+ * from @mdx-js/mdx back into mdx with the modifications. We
+ * don't want to maintain subtly different parsing code if we
+ * can avoid it.
+ */
+const hasDefaultExport = (str, options) => {
+  let hasDefaultExportBool = false;
+
+  function getDefaultExportBlock(subvalue) {
+    const isDefault = isExportDefault(subvalue);
+    hasDefaultExportBool = hasDefaultExportBool || isDefault;
+    return isDefault;
+  }
+  const tokenizeEsSyntax = (eat, value) => {
+    const index = value.indexOf(EMPTY_NEWLINE);
+    const subvalue = value.slice(0, index);
+
+    if (isExport(subvalue) || isImport(subvalue)) {
+      return eat(subvalue)({
+        type: isExport(subvalue) ? "export" : "import",
+        default: getDefaultExportBlock(subvalue),
+        value: subvalue
+      });
+    }
+  };
+
+  tokenizeEsSyntax.locator = (value, fromIndex) => {
+    return isExport(value) || isImport(value) ? -1 : 1;
+  };
+
+  function esSyntax() {
+    var Parser = this.Parser;
+    var tokenizers = Parser.prototype.blockTokenizers;
+    var methods = Parser.prototype.blockMethods;
+
+    tokenizers.esSyntax = tokenizeEsSyntax;
+
+    methods.splice(methods.indexOf("paragraph"), 0, "esSyntax");
+  }
+
+  const { content } = grayMatter(str);
+  const fn = unified()
+    .use(toMDAST, options)
+    .use(esSyntax)
+    .use(squeeze, options)
+    .parse(content)
+    .toString();
+
+  return hasDefaultExportBool;
+};
 
 module.exports = async function(content) {
   const callback = this.async();
@@ -29,7 +99,7 @@ module.exports = async function(content) {
   let code = content;
   // after running mdx, the code *always* has a default export, so this
   // check needs to happen first.
-  if (!hasDefaultExport(content) && !!defaultLayout) {
+  if (!hasDefaultExport(content, DEFAULT_OPTIONS) && !!defaultLayout) {
     debug("inserting default layout", defaultLayout);
     const { content: contentWithoutFrontmatter } = grayMatter(content);