Weighted keyframes are not properly evaluated in the particle system.

Repro:
- Open project in attachment.
- Open scene 'WeightedParticles'.
- Press play.

There are two particle systems in that scene. Left most particle system changes the 'Start Size' of the particles using an animation curve without weighted keys, while right most particle system uses weights to do the same. You may notice in playmode that both systems evaluate the same. This is wrong. Right most particle system should have a longer ease in time before particles grow.

This is an injection introduced with weighted keyframes in the animation curve.
Reproducible: 2018.1a5, 2018.2a1