RouterOutlet loads component 2x in a race condition

[tbosch]

RouterOutlet#reuse checks if there is no current _componentRef and in that case creates a new component.
However, _componentRef is not set until the call to DynamicComponentLoader.loadNextToLocation is finished (in RouterOutlet#activate). If that call takes a bit longer (e.g. loading an html file from the server) RouterOutlet will create 2 instances of the component.

On my branch for codegen templates that changes the timing of DynamicComponentLoading a bit, the test for syncRoutesWithDynamicComponents (in sync_route_spec_impl.ts) fails because of this.

[tbosch]

Setting to P1 as the issue the commit that introduced this logic was marked as P0... (#7318)

[naomiblack]

@mhevery can you figure out who should work on this one? It seems to be related to #7318. Tests for the codegen branch are failing because of this issue.

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