Add type safety for `global.SwaggerConverter`

[jimmycann]

Hi there,

Keep running into this error in a private repo, so I thought I'd add a small check

TypeError: Cannot read property 'convert' of undefined
    at Object.<anonymous> (...src/node_modules/swagger-tools/lib/specs.js:39:46)
    at Module._compile (module.js:635:30)

I haven't yet looked closer as to why swagger-tools is involved in my code to be honest. Swagger is turned off using the enabled: false flag. swagger-tools is a sub dependency of a private library I don't have a lot of control over, so I thought it would be easier to submit a patch here.

It looks like the reason that global variable is coming on for me as undefined is that in the referenced library, swagger-converter, it is declaring SwaggerConverter as a method on window, which I assume would work in the browser, but not in node.

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[jimmycann]

Signed the Google Developer CLA

[googlebot]

CLAs look good, thanks!