Circular type references in material2

[mprobst]

src/cdk/overlay/position/position-strategy.ts imports OverlayRef from src/cdk/overlay/overlay-ref.ts. That in turn imports src/cdk/overlay/overlay-config.ts, which in turn imports PositionStrategy, which in turn makes some of our tooling sad.

[mprobst]

@jelbourn can we merge crisbeto@41b77d7 to fix this? This is still blocking us in g4.

[jelbourn]

I'll try to take a look soon; the last time I tried to sync the fix it had a bunch of failing presubmits

[mprobst]

Thanks Jeremy!