Commits on Feb 11, 2017

1. Fix use of uninitialized memory when unsetting flex property …

   When a property on a node is set for the first time, we read and store
   the property's initial value before updating it. Later, if the property
   is nullified in JavaScript, we'll write the earlier-obtained initial
   value to the native representation.
   
   In order to obtain the initial value, there must be a property getter
   that corresponds to the setter. If no getter exists, the slot to store
   the initial value will remain silently uninitialized. If the property is
   nullified from JavaScript, we'll write the garbage value of that
   uninitialized variable into the native representation, and chaos will
   ensue.
   
   This commit makes four changes to resolve this issue:
   
   1. It adds an assertion that a getter exists every time a property is
      written. It would be better to enforce this statically, but I'm not
      aware of a means to accomplish this.
   
   2. It adds a getter for the `flex` property, which previously lacked
      one. Prior to this change, setting then removing the `flex` property
      on a view would result in layout glitches and redboxes about `NaN`
      and `Infinity` in the bound and offset of a view.
   
   3. It adds a getter for the `on` property for RCTSwitch, which can never
      be undefined, but is triggering the newly-added assertion.
   
   4. It adds an `unselectedItemTintColor` property for RCTTabBar, which
      appears to have been an oversight.
   
   There have been a number of complaints about transient redboxes with
   messages about illegal `NaN`s and `Infinity`s. Because this bug pertains
   to uninitialized memory which non-deterministic by nature, it's possible
   that a number of these difficult-to-repro issues will be resolved by
   this bugfix.

   

   jamesreggio committed Feb 11, 2017
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