[record_use] Correctness: Support nested const instances

[dcharkes]

Currently, the way that const instances are encoded is with a json map where the keys are the fields and the values are the field values. For the top-level instance you can look at the corresponding definition:

    {
      "definition": {
        "identifier": {
          "uri": "instance_class.dart",
          "name": "MyClass"
        },
        "loading_unit": "1"
      },
      "instances": [
        {
          "constant_index": 1,
          "loading_unit": "1",
          "@": 0
        }
      ]
    }

But for constants that are nested inside other constants, there is no way what the definition is.

  "constants": [
    {
      "type": "Instance",
      "value": {
        "i": 0
      }
    }
  ],

We should consider having a top-level list of definitions and letting the current place of definitions refer with an index into that list.