Support triple-quoted block strings

[jeroen]

@schloerke has found in our R bindings that the kitchen sink example produces invalid JSON:

curl -OL https://raw.githubusercontent.com/graphql/graphql-js/master/src/language/__tests__/kitchen-sink.graphql
./dump_json_ast kitchen-sink.graphql

The output has an invalid trailing comma (which you can validate on http://pro.jsonlint.com)

[swolchok]

Good catch! The null value is a special case that I missed because it has no fields besides the source location.

[dvergeylen]

Hello,

I am afraid the kitchen test changed a few months ago. Testing on a fresh clone I get the following output:

$ # Init
$ git clone https://github.com/graphql/libgraphqlparser
$ cd libgraphqlparser/
$ cmake .
$ make

$ # Test
$ curl -OL https://raw.githubusercontent.com/graphql/graphql-js/master/src/language/__tests__/kitchen-sink.graphql
$ ./dump_json_ast kitchen-sink.graphql
Parser failed with error: 49.58-59: Unterminated string

Am I doing something wrong?

Edit: diff between reference graphql-js test and hosted kitchen test:

 fragment frag on Friend {
-  foo(size: $size, bar: $b, obj: {key: "value"})
+  foo(size: $size, bar: $b, obj: {key: "value", block: """
+
+      block string uses \"""
+
+  """})
 }