IScriptSnapshot getChangeRange should allow undefined return value

[cgauld]

TypeScript Version: nightly

Expected behavior:
The typescriptServices.d.ts file signature for getChangeRange in the IScriptSnapshot interface should allow implementations to return undefined - as mentioned in its accompanying comment:

interface IScriptSnapshot {
       // [...omitted...]
        /**
         * Gets the TextChangeRange that describe how the text changed between this text and
         * an older version.  This information is used by the incremental parser to determine
         * what sections of the script need to be re-parsed.  'undefined' can be returned if the
         * change range cannot be determined.  However, in that case, incremental parsing will
         * not happen and the entire document will be re - parsed.
         */
        getChangeRange(oldSnapshot: IScriptSnapshot): TextChangeRange | undefined;

       // [...omitted...]
    }

Actual behavior:
It currently does not :-)

interface IScriptSnapshot {
       // [...omitted...]
        /**
         * Gets the TextChangeRange that describe how the text changed between this text and
         * an older version.  This information is used by the incremental parser to determine
         * what sections of the script need to be re-parsed.  'undefined' can be returned if the
         * change range cannot be determined.  However, in that case, incremental parsing will
         * not happen and the entire document will be re - parsed.
         */
        getChangeRange(oldSnapshot: IScriptSnapshot): TextChangeRange;

       // [...omitted...]
    }

I may be missing something however so apologies if I'm off the mark here!

Cheers.

[mhegazy]

Feel free to send a PR to update the declaration.

[DanielRosenwasser]

👍 @HerringtonDarkholme