Analyzer tests for package cache location failing on Windows

[whesse]

This bug was originally filed against pub, but the problem seems to be in the tests, not pub.
dart-lang/pub#1429

On the windows package test bots pkg-win-be and pkg-win-debug-be, on the Dart waterfall, tests are failing when they report absolute windows paths where a different absolute unix path is expected, as in the error message

FAIL: AbstractContextManagerTest | test_setRoots_addFolderWithPackagespec
Expected: '/home/somebody/.pub/cache/unittest-0.9.9/lib/unittest.dart'
Actual: 'E:\b\build\slave\pkg-win-be\build\sdk\home\somebody.pub\cache\unittest-0.9.9\lib\unittest.dart'
Which: is different.
Expected: /home/some ...
Actual: E:\b\bui ...
^
Differ at offset 0

The tests that fail are:
pkg/analysis_server/test/context_manager_test
analysis_server/test/context_manager_test:
analyzer/test/generated/source_factory_test
analyzer/test/src/context/builder_test
analyzer/test/src/summary/linker_test
analyzer/test/src/summary/prelinker_test
analyzer/test/src/summary/summarize_elements_strong_test
analyzer/test/src/summary/summarize_elements_test

@stereotype441

Master issue is #26828