"If you or your agent or exclusive licensee institute or order or agree to the institution of patent litigation against any entity (including a cross-claim or counterclaim in a lawsuit) alleging that this implementation of Polymer or any code incorporated within this implementation of Polymer constitutes direct or contributory patent infringement, or inducement of patent infringement, then any patent rights granted to you under this License for this implementation of Polymer shall terminate as of the date such litigation is filed."
Polymer license only concerns itself with itself. FB license concerns all patents you and FB might have in addition to patents/righs concerning the react code itself.
There is a big difference, although it might not look like that.
"against any entity (including a cross-claim or counterclaim in a lawsuit) alleging that THIS IMPLEMENTATION OF POLYMER or OR ANY CODE INCORPORATED WITHIN this implementation of Polymer".
I know I ask much of you but you can do it.
BTW. congratulations on copy pasting the text I pasted above...
"If you or your agent or exclusive licensee institute or order or agree to the institution of patent litigation against any entity (including a cross-claim or counterclaim in a lawsuit) alleging that this implementation of Polymer or any code incorporated within this implementation of Polymer constitutes direct or contributory patent infringement, or inducement of patent infringement, then any patent rights granted to you under this License for this implementation of Polymer shall terminate as of the date such litigation is filed."