[front end] Take over responsibility for tracking expression info.

Shifts the responsibility for tracking the flow analysis
`ExpressionInfo` objects from the shared `FlowAnalysis` class to the
analyzer's `FlowAnalysisHelper` class.

This is a step towards a long term plan for the front end to pass
these `ExpressionInfo` objects around on the stack: the code that
resolves an expression will return that expression's `ExpressionInfo`
(among other data), and the code that resolves the expression's parent
will plumb the data to the appropriate flow analysis method. This
should be much less error prone than the current approach of keeping
track of a map from `Expression` to `ExpressionInfo`, since we won't
have to be careful about updating the map when rewriting the AST. It
should also be more efficient, since it will avoid costly map lookups.

Change-Id: I825f32aa226360514075fbb1a20d503a6a6a6964
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/518980
Reviewed-by: Chloe Stefantsova <cstefantsova@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>