Browser implementation workarounds

Workarounds

Closure works around a number of browser implementation issues.

Edge

• ChakraCore #1496 is an optimizer bug that causes constructor invocations to return the class instead of the instance. Closure Library explicitly checks the user agent for Edge and considers it a non-ES6-compliant browser as a result.
• ChakraCore #3217 is a bug in Reflect.construct that prevents the compiler's super() call transpilation from working correctly. The compiler checks for this bug and uses a slower polyfill that calls Reflect.setPrototypeOf instead.

Safari

• TODO(@concavelenz): eval scope issues, etc