[json] support JSON schema draft-06

[itowlson]

JSON Schema draft-06 includes a const keyword to indicate that a property must be a specific value (http://json-schema.org/latest/json-schema-validation.html#rfc.section.6.24). VS Code does not report a problem if the property does not match the value. (It does work if you use an enum with a single value.)

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Steps to Reproduce:

1. See sample repo https://github.com/itowlson/vscode-json-schema-test and load the extension. Note the contents of schema/foo.schema.json.

2. Run this extension in the Extension Development Host (or create a new extension and add the jsonValidation section and foo.schema.json file from the sample repo).

3. In the Extension Development Host, open the file test/test.foo.json.

4. View the "apiVersion" element. Expected: A warning squiggly, and hover message along the lines of "Value is not accepted. Valid value: ''2017-01-01.1.0'" (basing this on the enum message - I'm sure you can word it better). Actual: no warning.

(The "apiVersionWorkaround" element works correctly - I included it only so you could compare the const behaviour to the single-value enum behaviour.)

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• VSCode Version: Code 1.13.1 (379d2ef, 2017-06-14T18:21:47.485Z)
• OS Version: Windows_NT ia32 10.0.14393
• Extensions:

Extension	Author	Version
jshint	dbaeumer	0.10.15
vscode-eslint	dbaeumer	1.2.11
mdhelper	joshbax	0.0.9

[lijunle]

Here are the supporting JSON schema keywords.

I use the pattern keyword with type:string as a workaround although they are not exact the same function.

Another missing keyword is contains.

[aeschli]

To verify: See http://json-schema.org/draft-06/json-schema-migration-faq.html for additions & changes:

property	change	--
"exclusiveMinimum"and "exclusiveMaximum"	changed from a boolean to a number to be consistent with the principle of keyword independence	wherever one of these would be true before, change the value to the corresponding "minimum" or "maximum" value and remove the "minimum"/"maximum"keyword
booleans as schemas	allowable anywhere, not just "additionalProperties"and "additionalItems"	true is equivalent to {}, false is equivalent to {"not": {}}, but the intent is more clear and implementations can optimize these cases more easily
"propertyNames"	added	takes a schema which validates the namesof all properties rather than their values
"contains"	added	array keyword that passes validation if its schema validates at least one array item
"const"	added	more readible form of a one-element "enum"

draf06test.schema.json

{
    "$schema": "http://json-schema.org/draft-06/schema",
    "type": "object",
    "properties": {
        "n": {
            "type": "number",
            "exclusiveMinimum": 5,
            "exclusiveMaximum": 7
        },
        "p": {
            "type": "object",
            "propertyNames": {
                "pattern": "[1-9]"
            }
        },
        "a": {
            "type": "array",
            "contains": {
                "type": "boolean"
            }
        },
        "foo": {
            "type": "string",
            "const": "bar",
            "description": "It can only be bar"
        }
    }
}

draft06test.json

{
    "$schema": "./draft06test.schema.json",
    "n": 5,
    "p": {
        "1": true,
        "f": false
    },
    "a": [ 1, 2],
    "foo": "bar"
}