fix(cdk): prevent attribute renaming in observe-content.ts for closure compiler ADVANCED optimizations

[gregmagolan]

I got the following error after building my new project with Angular Material 2.0.0-beta.12, Angular 5.0.0-rc.0 & the latest closure compiler.

    at j.(anonymous function).XMLHttpRequest.t.responseBlob.e.function.o.(anonymous function).j.(anonymous function).(anonymous function) [as observe] (http://localhost:8888/assets/js/zone.min.js:1:1959)
    at $ObserveContent$$module$node_modules$$angular$cdk$esm2015$observers$$.$$ObserveContent$$module$node_modules$$angular$cdk$esm2015$observers$$$$$ngAfterContentInit$$ [as $ngAfterContentInit$] (http://localhost:8888/assets/js/browser.js:7727:27)
    at $callProviderLifecycles$$module$node_modules$$angular$core$esm2015$core$$ (http://localhost:8888/assets/js/browser.js:3545:269)
    at $callLifecycleHooksChildrenFirst$$module$node_modules$$angular$core$esm2015$core$$ (http://localhost:8888/assets/js/browser.js:3534:87)
    at $checkAndUpdateView$$module$node_modules$$angular$core$esm2015$core$$ (http://localhost:8888/assets/js/browser.js:3845:3)
    at $callViewAction$$module$node_modules$$angular$core$esm2015$core$$ (http://localhost:8888/assets/js/browser.js:4208:67)
    at $execComponentViewsAction$$module$node_modules$$angular$core$esm2015$core$$ (http://localhost:8888/assets/js/browser.js:4178:49)
    at $checkAndUpdateView$$module$node_modules$$angular$core$esm2015$core$$ (http://localhost:8888/assets/js/browser.js:3847:3)
    at $callViewAction$$module$node_modules$$angular$core$esm2015$core$$ (http://localhost:8888/assets/js/browser.js:4208:67)
    at $execComponentViewsAction$$module$node_modules$$angular$core$esm2015$core$$ (http://localhost:8888/assets/js/browser.js:4178:49)

Tracked it down to attributes in an observe(...) call in observer-content.ts being renamed during advanced optimizations.

This PR fixes the issue.

[jelbourn]

LGTM

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