fix(stitch) fix abstract type merge failure

[gmac]

Fixes the bug discussed in: #1973. This appears to be a regression that appeared in a recent version. This applies the fix mentioned by @yaacovCR, and adds supporting tests.

    ReferenceError: Cannot access 'typeMap' before initialization

      208 |   const typeMap = configs.reduce((acc, config) => {
      209 |     config.types.forEach(type => {
    > 210 |       typeMap[type.name] = type;
          |       ^
      211 |     });
      212 |     return acc;
      213 |   }, Object.create(null));

      at packages/stitch/src/mergeCandidates.ts:210:7
          at Array.forEach (<anonymous>)
      at packages/stitch/src/mergeCandidates.ts:209:18
          at Array.reduce (<anonymous>)
      at mergeUnionTypeCandidates (packages/stitch/src/mergeCandidates.ts:208:27)
      at Object.mergeCandidates (packages/stitch/src/mergeCandidates.ts:45:12)
      at packages/stitch/src/typeCandidates.ts:212:27
          at Array.forEach (<anonymous>)
      at Object.buildTypeMap (packages/stitch/src/typeCandidates.ts:202:31)
      at Object.stitchSchemas (packages/stitch/src/stitchSchemas.ts:121:19)

TODO:

• If this PR is a new feature, reference an issue where a consensus about the design was reached (not necessary for small changes)
• Make sure all of the significant new logic is covered by tests
• Rebase your changes on master so that they can be merged easily
• Make sure all tests and linter rules pass

[yaacovCR]

Looks like regression introduces in #1917 and releases in v6.1.0. Thanks for catching this!

[theguild-bot]

The latest changes of this PR are available as alpha in npm: 6.2.1-alpha-3f01f855.0

Quickly update your package.json by running:

npx match-version @graphql-tools 6.2.1-alpha-3f01f855.0