v5 polymorphism branch: @stream bug

[jemgillam]

Ref more-tests branch
Ref grafast/grafast/__tests__/dcc/queries/consumable-items-stream2.test.graphql

A possible bug in the way @stream is used, or a possible bug in our new test suite

Given the query

query ConsumableItems {
  crawler(id: 101) {
    id
    ... on HasInventory {
      items(first: 3) {
        __typename
        id
        ... on HasContents {
          contents(first: 3) {
            __typename
            id
            ... on Equipment {
              maxDurability
            }
            ... on Consumable {
              effect
              ... ...

@stream works as expected when placed here:

query ConsumableItemsStream @incremental {
  crawler(id: 101) {
    id
    ... on HasInventory {
      items(first: 3) @stream {
        __typename
        id 
        ... ...

but when placed in a nested list it is returning nulls:

query ConsumableItemsStream2 @incremental {
  crawler(id: 101) {
    id
    ... on HasInventory {
      items(first: 3) {
        __typename
        id
        ... on HasContents {
          contents(first: 3) @stream {
            __typename
            id
            ... ...

returns the following from our test suite:

Error: Expected operation to be incremental

AssertionError: expected '{\n  crawler: {\n    id: 101,\n    na…' to equal '{\n  crawler: {\n    id: 101,\n    na…'
      + expected - actual

                 name: 'Dolores',
               },
               effect: 'Soften impact surface. Impact x 5',
               contents: [
      -          null,
      -          null,
      +          {
      +            __typename: 'Consumable',
      +            id: 207,
      +            name: 'Crowd Blast Potion',
      +            effect: 'Imitates the Crowd Blast skill',
      +          },
      +          {
      +            __typename: 'Consumable',
      +            id: 208,
      +            name: 'Rock Buffalo Potion',
      +            effect: "A required component of Dolores Doesn't Splat Potion",
      +          },
               ],
             },

[jemgillam]

More context, for the working @stream queries, a .incsig file is generated showing data about the streamed payloads. No such file is being generated for this query.