test for memory leaks

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Looks like the sim is leaking memory. With ?fuzzMouse, heap snapshots spaced a few minutes apart had the following sizes:

Snapshot 1: 50.1 MB
Snapshot 2: 86.1 MB
Snapshot 3: 98.0 MB
Snapshot 4: 164 MB
Snapshot 5: 231 MB
Snapshot 6: 264 MB
Snapshot 7: 330 MB

Mac OS X 10.11.6, Chrome 56.0.2924.87.

Unfortunately the summary shows that what is growing is Array, (array) and Object, so not immediately obvious what is leaking. RectangularPushButton is also leaking.

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Majority of the leaks are plugged. Over a similar period (15 minutes) with ?fuzzMouse, here are the new heap snapshots:

Snapshot 1: 45.0 MB
Snapshot 2: 45.5 MB
Snapshot 3: 46.4 MB
Snapshot 4: 49.2 MB
Snapshot 5: 49.2 MB
Snapshot 6: 51.8 MB
Snapshot 7: 51.6 MB

... and subsequent snapshots seemed to stabilize around 53 MB.

I'll continue to look for leaks. But an 8MB increase over 15 minutes of fuzz testing is probably reasonable. Users are unlikely to exercise the simulation that much in hours of actual use.

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After another 10 minutes of fuzz testing (25 minutes total), the heap size is only 55 MB. And there's nothing that appears to be growing in the Summary view. So I'm going to call this done.

Other commits that were related to fixing this:
@d18e84f9d1108ca791a7ca75b5a4186206c2e925
@e9a19816f65ec1bc475fb4010dca5d4a6b819c3f
@5f4caf94cbaa48a75fdf3fc71b749b319ef4b145
@4d6da695b1a65cec3a5297516035745350ebf33c