[SPIKE] Trying a ground up rebuild of the printer to see what I can learn

CHANGELOG.unreleased.md

@@ -146,8 +146,31 @@ Previously, Prettier would sometimes ignore whitespace when formatting comments.
 </div>
 ```
 
+### Handlebars: Improve comment formatting ([#6234] by [@gavinjoyce])
+
+Previously, Prettier would incorrectly decode HTML entiites.
+
+<!-- prettier-ignore -->
+```hbs
+// Input
+<p>
+  Some escaped characters: &lt; &gt; &amp;
+</p>
+
+// Output (Prettier stable)
+<p>
+  Some escaped characters: < > &
+</p>
+
+// Output (Prettier master)
+<p>
+  Some escaped characters: &lt; &gt; &amp;
+</p>
+```
+
 [#6209]: https://github.com/prettier/prettier/pull/6209
 [#6186]: https://github.com/prettier/prettier/pull/6186
 [#6186]: https://github.com/prettier/prettier/pull/6206
+[#6234]: https://github.com/prettier/prettier/pull/6234
 [@duailibe]: https://github.com/duailibe
 [@gavinjoyce]: https://github.com/gavinjoyce

package.json

@@ -17,7 +17,7 @@
     "@angular/compiler": "7.2.9",
     "@babel/code-frame": "7.0.0",
     "@babel/parser": "7.2.0",
-    "@glimmer/syntax": "0.39.3",
+    "@glimmer/syntax": "0.41.0",
     "@iarna/toml": "2.2.3",
     "@typescript-eslint/typescript-estree": "1.10.2",
     "angular-estree-parser": "1.1.5",

src/language-handlebars/parser-glimmer.js

@@ -8,7 +8,8 @@ function parse(text) {
     return glimmer(text, {
       plugins: {
         ast: []
-      }
+      },
+      mode: "codemod"
     });
     /* istanbul ignore next */
   } catch (error) {