Fix #4464: backticked expressions in class body (#4712)

* Fix #4464: backticked expressions in class body should be left in the body, not hoisted

* Fix #4464: backticked expressions in class body should be left in the body, not hoisted

* Simplify fix for #4464

This uses more of the existing machinery for moving class body
expressions into the initializer.

* Clarify the purpose of Class::addInitializerExpression

* Further clarify the purpose of Class::addInitializerExpression

* Add reference to class fields; format
 comment wrapping

* Reapply 1d3af8c, that got lost because of rebase/force-push shenanigans

* Updated output

src/nodes.coffee

@@ -1752,10 +1752,23 @@ exports.Class = class Class extends Base
 
   # Add an expression to the class initializer
   #
-  # NOTE Currently, only methods and static methods are valid in ES class initializers.
-  # When additional expressions become valid, this method should be updated to handle them.
+  # This is the key method for determining whether an expression in a class
+  # body should appear in the initializer or the executable body. If the given
+  # `node` is valid in a class body the method will return a (new, modified,
+  # or identical) node for inclusion in the class initializer, otherwise
+  # nothing will be returned and the node will appear in the executable body.
+  #
+  # At time of writing, only methods (instance and static) are valid in ES
+  # class initializers. As new ES class features (such as class fields) reach
+  # Stage 4, this method will need to be updated to support them. We
+  # additionally allow `PassthroughLiteral`s (backticked expressions) in the
+  # initializer as an escape hatch for ES features that are not implemented
+  # (e.g. getters and setters defined via the `get` and `set` keywords as
+  # opposed to the `Object.defineProperty` method).
   addInitializerExpression: (node) ->
-    if @validInitializerMethod node
+    if node.unwrapAll() instanceof PassthroughLiteral
+      node
+    else if @validInitializerMethod node
       @addInitializerMethod node
     else
       null

test/classes.coffee

@@ -1833,3 +1833,18 @@ test "#4591: super.x.y, super['x'].y", ->
   eq 2, b.t
   eq 2, b.s
   eq 2, b.r
+
+test "#4464: backticked expressions in class body", ->
+  class A
+    `get x() { return 42; }`
+
+  class B
+    `get x() { return 42; }`
+    constructor: ->
+      @y = 84
+
+  a = new A
+  eq 42, a.x
+  b = new B
+  eq 42, b.x
+  eq 84, b.y