Dart2: Stop relying on AnalysisContext-only APIs

[matanlurey]

In Dart2's move to a unified CFE (potentially AnalysisDriver first, then CFE), we can no longer use analyzer APIs in our compiler that are only supported in AnalysisContext (also known elsewhere as the "Task Model").

Unfortunately it's hard for me to know if we have violations (I know computeNode is one).

Known violations:

• Use of the computeNode API: Removing uses of "computeNode()" from the compiler. #801.

[matanlurey]

Covered in #801 for now.