PIC32MX Timer 1 Interrupt on Uno32
_ISR macro adds an interrupt if and only if the Multi-vector mode is enabled.
So to enable it use 

INTCONSET = _INTCON_MVEC_MASK;

Otherwise, to go with Single vector mode use _ISR_SINGLE__ instead as following 

void __ISR_SINGLE__ Handler(void) {
    if(IFS0bits.T1IF){
    LATFbits.LATF0 = ~LATFbits.LATF0;

    IFS0bits.T1IF = 0; // Clear interrupt flag
   }
}