StatementedNode uses inconsistent source for child count

[dsherret]

I noticed when inserting incorrect syntax:

export type compilerVersions = "2.8.1" | "2.6.2" | "2.7.2";

export enum CompilerVersion {
    typescript = "2.8.1",
    typescript-2.6.2 = "2.6.2",
    typescript-2.7.2 = "2.7.2"
}

That when then calling:

sourceFile.addFunction({...});

It would throw this error:

V:\ts-ast-viewer\node_modules\ts-simple-ast\dist\errors\BaseError.js:16
        var _this = _super.call(this, message) || this;
                           ^
Error: Invalid index: The max index is 5, but 6 was specified.
    at InvalidOperationError.BaseError [as constructor] (V:\ts-ast-viewer\node_modules\ts-simple-ast\dist\errors\BaseError.js:16:28)
    at new InvalidOperationError (V:\ts-ast-viewer\node_modules\ts-simple-ast\dist\errors\InvalidOperationError.js:17:28)
    at Object.verifyAndGetIndex (V:\ts-ast-viewer\node_modules\ts-simple-ast\dist\manipulation\helpers\verifyAndGetIndex.js:14:15)
    at Object.insertIntoBracesOrSourceFileWithGetChildren (V:\ts-ast-viewer\node_modules\ts-simple-ast\dist\manipulation\manipulations\insertion.js:135:27)
    at SourceFile.StatementedNode.class_1._insertChildren (V:\ts-ast-viewer\node_modules\ts-simple-ast\dist\compiler\statement\StatementedNode.js:397:35)
    at SourceFile.StatementedNode.class_1.insertFunctions (V:\ts-ast-viewer\node_modules\ts-simple-ast\dist\compiler\statement\StatementedNode.js:168:25)
    at SourceFile.StatementedNode.class_1.addFunctions (V:\ts-ast-viewer\node_modules\ts-simple-ast\dist\compiler\statement\StatementedNode.js:161:25)
    at SourceFile.StatementedNode.class_1.addFunction (V:\ts-ast-viewer\node_modules\ts-simple-ast\dist\compiler\statement\StatementedNode.js:158:25)
    at Object.<anonymous> (V:\ts-ast-viewer\scripts\createCompilerVersions.ts:35:22)
    at Module._compile (module.js:624:30)

The problem is that StatementedNode uses one source for the child count in the add methods and a different one when verifying the index is valid.