Function parameter defaults cannot depend on prior argument values, regression compared to other implementations

[HT154]

local test = function(a, b=a.y) a.x + b;

[
  test({x: 'x', y: 'y'}),
  test({x: 'x', y: 'y'}, 'b'),
  test({x: 'x'}, 'b'),
]

Results:

$ jsonnet test.jsonnet                                                        
[
   "xy",
   "xb",
   "xb"
]

$ go-jsonnet test.jsonnet
[
   "xy",
   "xb",
   "xb"
]

$ jrsonnet test.jsonnet                                                        
variable is not defined: a
    /<path>/test.jsonnet:3:28-30: variable <a>
    /<path>/test.jsonnet:3:39-41: variable <b>
    /<path>/test.jsonnet:6:3-26 : function <test> call