fix(zone.js): restore definition of global

[alexeagle]

This partially reverts some changes from angular/zone.js@71b9371#diff-dd469785fca8680a5b33b1e81c5cfd91R1420
These broke the g3sync of zone.js because we use the output of the TypeScript compiler directly, rather than rely on the rollup commonjs plugin to define the global symbol

[alexeagle]

FYI the resulting change in the bundle looks like

alexeagle@alexeagle:~/Projects/angular$ diff /tmp/oldzone/package/dist/zone.js dist/bin/packages/zone.js/npm_package/dist/zone.js
11d10
<     var commonjsGlobal = typeof window !== 'undefined' ? window : typeof global !== 'undefined' ? global : typeof self !== 'undefined' ? self : {};
651c650
<     })(commonjsGlobal);
---
>     })(typeof window !== 'undefined' && window || typeof self !== 'undefined' && self || global);

[JiaLiPassion]

Thanks, LGTM.

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