Exclude unused standard types from the schema

[spawnia]

Improves our behaviour in regards to #964, except Schema::getType()

[spawnia]

Should Introspection::fromSchema() throw on errors?

[codecov]

Codecov Report

Merging #974 (cb236ae) into master (694088f) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master     #974   +/-   ##
=========================================
  Coverage     94.27%   94.27%           
  Complexity       50       50           
=========================================
  Files           118      118           
  Lines          9577     9579    +2     
=========================================
+ Hits           9029     9031    +2     
  Misses          548      548           

Impacted Files	Coverage Δ
src/Type/Introspection.php	99.69% <100.00%> (+<0.01%)	⬆️
src/Type/Schema.php	88.88% <100.00%> (+0.06%)	⬆️
src/Utils/BuildClientSchema.php	100.00% <100.00%> (ø)
src/Utils/TypeInfo.php	95.67% <100.00%> (ø)

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[vhenzl]

Schema::getType() should return null for unused standard types (graphql-js returns undefined): https://github.com/graphql/graphql-js/blob/v15.6.1/src/utilities/__tests__/buildASTSchema-test.js#L161-L168

There is a test for it in 33c6bc1 (#928) you can copy here.

[spawnia]

Thanks @vhenzl for bringing that up. I noticed that difference before and changed the implementation of a test to use getTypeMap() instead.

I documented the difference for now (see #964 (comment)) and plan to leave the issue open until we can resolve it fully. Unless you have an idea how to fix our implementation, I would like to go ahead with this PR. It does at least fix the externally observable behaviour in regards to introspection.

[vhenzl]

Right, makes sense.