Fix shorthands after interpolated key in objects

Fixes #4324.

lib/coffee-script/nodes.js

@@ -1488,6 +1488,9 @@
               value = prop.value;
             } else {
               ref3 = prop.base.cache(o), key = ref3[0], value = ref3[1];
+              if (key instanceof IdentifierLiteral) {
+                key = new PropertyName(key.value);
+              }
             }
             prop = new Assign(new Value(new IdentifierLiteral(oref), [new Access(key)]), value);
           }

src/nodes.coffee

@@ -1019,6 +1019,7 @@ exports.Obj = class Obj extends Base
             value = prop.value
           else
             [key, value] = prop.base.cache o
+            key = new PropertyName key.value if key instanceof IdentifierLiteral
           prop = new Assign (new Value (new IdentifierLiteral oref), [new Access key]), value
       if indent then answer.push @makeCode indent
       answer.push prop.compileToFragments(o, LEVEL_TOP)...

test/objects.coffee

@@ -569,3 +569,9 @@ test "object keys with interpolations", ->
     interpolated:
       nested:
         123: 456
+
+test "#4324: Shorthand after interpolated key", ->
+  a = 2
+  obj = {"#{1}": 1, a}
+  eq 1, obj[1]
+  eq 2, obj.a