Incorrect formatter behavior: missing space after opening template string brace

[cspotcode]

TypeScript Version: 2.7.0-dev.20171020

Code

const a = `
    ${123}${123}
`;

With formatting option "insertSpaceAfterOpeningAndBeforeClosingTemplateStringBraces" set to "true." I did this in VSCode with:

{"typescript.format.insertSpaceAfterOpeningAndBeforeClosingTemplateStringBraces": true}

Open the .ts file in VSCode and run the "Format document" command. (If requested, I can make a reproduction that uses TypeScript's formatting API directly)

Expected behavior:

"Format document" inserts four spaces adjacent to the opening and closing curly braces:

const a = `
    ${ 123 }${ 123 }
`;

Actual behavior:

There is no whitespace before the first 123.

const a = `
    ${123 }${ 123 }
`;

[cspotcode]

I think I've traced the problem to this bit of code:
https://github.com/Microsoft/TypeScript/blob/master/src/services/formatting/formatting.ts#L1139-L1142

The rule to insert a space is being skipped because previousStartLine is different than currentStartLine. However, in this particular situation, the newline occurs within the templateHead token, so it really shouldn't be skipping the rule.