Fix failure to get source maps on windows with base change in index.html

[annagrin]

Use only forward slashes for server paths in FrontendServerRequireStrategy.

Server path should only contain forward slashes, but p.join() added backward slashes on windows when prepending a base path to the server path. This caused dwds to use incorrect source map paths and a failures to set breakpoints.

Changes:

• use string concatenation instead of p.join in FrontendServerRequireStrategy.
• Add breakpoint and evaluation tests for non-trivial bases.
• Move out shared code for evaluation tests for various compilation modes.

Closes: #1624
Helps: flutter/flutter-intellij#6159