targets:wasm_exec.js set _pendingEvent to null

[soypat]

// syscall/js.handleEvent reads _pendingEvent and returns early only when
// it IsNull(). Leaving it `undefined` is not null, so handleEvent falls
// through to cb.Get("id") and panics with "call of Value.Get on
// undefined", which surfaces as RuntimeError: unreachable. Upstream Go's
// wasm_exec.js initializes this in its constructor; this line restores
// parity. Any resume() that runs without a pending event -- and resume()
// always spawns a handleEvent goroutine -- depends on it.

[dgryski]

LGTM

[github-actions]

Size difference with the dev branch:

Binary size difference

 flash                          ram
 before   after   diff          before   after   diff
  19684   19684      0   0.00%    7052    7052      0   0.00% tinygo build -size short -o ./build/test.hex -target=feather-rp2040 ./examples/adafruit4650
  63228   63228      0   0.00%    6788    6788      0   0.00% tinygo build -size short -o ./build/test.hex -target=itsybitsy-m0 ./examples/adt7410/main.go
  10024   10024      0   0.00%    5348    5348      0   0.00% tinygo build -size short -o ./build/test.hex -target=itsybitsy-m0 ./examples/adxl345/main.go
  14628   14628      0   0.00%    7396    7396      0   0.00% tinygo build -size short -o ./build/test.hex -target=pybadge ./examples/amg88xx
  10184   10184      0   0.00%    5348    5348      0   0.00% tinygo build -size short -o ./build/test.hex -target=itsybitsy-m0 ./examples/apa102/main.go
  12592   12592      0   0.00%    7172    7172      0   0.00% tinygo build -size short -o ./build/test.hex -target=nano-33-ble ./examples/apds9960/proximity/main.go
  11136   11136      0   0.00%    5360    5360      0   0.00% tinygo build -size short -o ./build/test.hex -target=itsybitsy-m0 ./examples/apa102/itsybitsy-m0/main.go
   7588    7588      0   0.00%    2312    2312      0   0.00% tinygo build -size short -o ./build/test.hex -target=microbit ./examples/at24cx/main.go
   9272    9272      0   0.00%    5340    5340      0   0.00% tinygo build -size short -o ./build/test.hex -target=itsybitsy-m0 ./examples/bh1750/main.go
   8660    8660      0   0.00%    5340    5340      0   0.00% tinygo build -size short -o ./build/test.hex -target=itsybitsy-m0 ./examples/blinkm/main.go
  71408   71408      0   0.00%    3656    3656      0   0.00% tinygo build -size short -o ./build/test.hex -target=pinetime     ./examples/bma42x/main.go
  67120   67120      0   0.00%    6804    6804      0   0.00% tinygo build -size short -o ./build/test.hex -target=itsybitsy-m0 ./examples/bmi160/main.go
  28952   28952      0   0.00%    5380    5380      0   0.00% tinygo build -size short -o ./build/test.hex -target=itsybitsy-m0 ./examples/bmp180/main.go
  65720   65720      0   0.00%    6828    6828      0   0.00% tinygo build -size short -o ./build/test.hex -target=itsybitsy-m0 ./examples/bmp280/main.go
  13080   13080      0   0.00%    5412    5412      0   0.00% tinygo build -size short -o ./build/test.hex -target=trinket-m0 ./examples/bmp388/main.go
  24024   24024      0   0.00%    6220    6220      0   0.00% tinygo build -size short -o ./build/test.hex -target=metro-rp2350 ./examples/bno08x/i2c/main.go
   8008    8008      0   0.00%    3344    3344      0   0.00% tinygo build -size short -o ./build/test.hex -target=bluepill ./examples/ds1307/sram/main.go
  22116   22116      0   0.00%    3548    3548      0   0.00% tinygo build -size short -o ./build/test.hex -target=bluepill ./examples/ds1307/time/main.go
  30556   30556      0   0.00%    5576    5576      0   0.00% tinygo build -size short -o ./build/test.hex -target=itsybitsy-m0 ./examples/ds3231/alarms/main.go
  44492   44492      0   0.00%    5576    5576      0   0.00% tinygo build -size short -o ./build/test.hex -target=itsybitsy-m0 ./examples/ds3231/basic/main.go
   4592    4592      0   0.00%    2272    2272      0   0.00% tinygo build -size short -o ./build/test.hex -target=microbit ./examples/easystepper/main.go
  71884   71884      0   0.00%    7588    7588      0   0.00% tinygo build -size short -o ./build/test.hex -target=itsybitsy-m0 ./examples/flash/console/spi
  67996   67996      0   0.00%    9628    9628      0   0.00% tinygo build -size short -o ./build/test.hex -target=pyportal ./examples/flash/console/qspi
   7128    7128      0   0.00%    2276    2276      0   0.00% tinygo build -size short -o ./build/test.hex -target=microbit ./examples/gc9a01/main.go
 694072  694072      0   0.00%  131564  131564      0   0.00%

[deadprogram]

Thanks for the correction @soypat and to @dgryski for review. Now merging.