`Object.assign` is not available in all supported browsers and one had to
provide a polyfill. This commit replaces `Object.assign` with the spread
operator (`...`), which TypeScript will transpile to ES5-compatible code.
Destructuring of the form:
function foo({a, b}: {a?, b?} = {})
breaks strictNullChecks, due to the TypeScript bug https://github.com/microsoft/typescript/issues/10078.
This change eliminates usage of destructuring in function argument lists in cases where it would leak
into the public API .d.ts.
