Fix interleaved comments in class decorators (#2661)

I wrote this fix a while ago but it conflicted with the class heuristic, now that #2660 fixes it, we can ship this one as well!

Fixes #1460
Fixes #2507

src/comments.js

@@ -192,6 +192,12 @@ function attach(comments, ast, text) {
           comment
         ) ||
         handleTryStatementComments(enclosingNode, followingNode, comment) ||
+        handleClassComments(
+          enclosingNode,
+          precedingNode,
+          followingNode,
+          comment
+        ) ||
         handleImportSpecifierComments(enclosingNode, comment) ||
         handleObjectPropertyComments(enclosingNode, comment) ||
         handleForComments(enclosingNode, precedingNode, comment) ||
@@ -246,6 +252,12 @@ function attach(comments, ast, text) {
           followingNode,
           comment
         ) ||
+        handleClassComments(
+          enclosingNode,
+          precedingNode,
+          followingNode,
+          comment
+        ) ||
         handleLabeledStatementComments(enclosingNode, comment) ||
         handleCallExpressionComments(precedingNode, enclosingNode, comment) ||
         handlePropertyComments(enclosingNode, comment) ||
@@ -551,6 +563,32 @@ function handleObjectPropertyAssignment(enclosingNode, precedingNode, comment) {
   return false;
 }
 
+function handleClassComments(
+  enclosingNode,
+  precedingNode,
+  followingNode,
+  comment
+) {
+  if (
+    enclosingNode &&
+    (enclosingNode.type === "ClassDeclaration" ||
+      enclosingNode.type === "ClassExpression") &&
+    (enclosingNode.decorators && enclosingNode.decorators.length > 0) &&
+    !(followingNode && followingNode.type === "Decorator")
+  ) {
+    if (!enclosingNode.decorators || enclosingNode.decorators.length === 0) {
+      addLeadingComment(enclosingNode, comment);
+    } else {
+      addTrailingComment(
+        enclosingNode.decorators[enclosingNode.decorators.length - 1],
+        comment
+      );
+    }
+    return true;
+  }
+  return false;
+}
+
 function handleMethodNameComments(text, enclosingNode, precedingNode, comment) {
   // This is only needed for estree parsers (flow, typescript) to attach
   // after a method name:

tests/decorators/__snapshots__/jsfmt.spec.js.snap

@@ -25,6 +25,13 @@ class X {
   ],
 })
 export class AppModule {}
+
+// A
+@Foo()
+// B
+@Bar()
+// C
+export class Bar{}
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 var x = 100;
 
@@ -48,6 +55,13 @@ class X {}
 })
 export class AppModule {}
 
+// A
+@Foo()
+// B
+@Bar()
+// C
+export class Bar {}
+
 `;
 
 exports[`methods.js 1`] = `

tests/decorators/comments.js

@@ -22,3 +22,10 @@ class X {
   ],
 })
 export class AppModule {}
+
+// A
+@Foo()
+// B
+@Bar()
+// C
+export class Bar{}