Add ES2018 as target to the used tsconfig-schema.json

[omniproc]

• VSCode Version: Code 1.19.2 (490ef76, 2018-01-10T15:55:03.538Z)
• OS Version: Windows_NT x64 10.0.14393

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vscode will download the tsconfig-schema.json used for tsconfig validation, which is awesome.
TS 2.7 just added support for ES2018 targets, see microsoft/TypeScript#20385

the current tsconfig-schema yet doesn't allow to use this. I'm not sure where you're pulling the schema from otherwise I'd added a pull request myself.

P.S.: note that with json-schema draft-07 if/then/else was added for even more flexible dependencies in the schema definition. E.g. "only allow target ES2018 if TS version is >=2.7" would be possible now.

[mjbvz]

Fixed upstream: SchemaStore/schemastore#404 (comment)