[flutter_tools] Eliminate globals.dart from flutter_plugins.dart - #194062

Reland flutter#193776, reverted by flutter#195635. Match CanvasKit requests by URI path
segments before constructing filesystem paths, preserving containment
checks and POSIX/Windows regression coverage.

Pass MustacheTemplateRenderer to FlutterWebPlatform.start in the regression
tests to satisfy the required templateRenderer parameter added by flutter#194062.

…flutter#195770)

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Relands flutter#193776, reverted by flutter#195635.

On Windows, converting a CanvasKit request URI to a filesystem path
before matching `canvaskit/` causes valid asset requests to return HTTP
404. Match URI path segments first, then construct and validate the
filesystem path. Restore regression coverage for POSIX and Windows
paths, including JavaScript/Wasm assets, response bodies and headers,
and invalid paths.

The regression tests now pass `MustacheTemplateRenderer` to
`FlutterWebPlatform.start`, addressing the required `templateRenderer`
parameter introduced by flutter#194062 that caused the original tree breakage.

Fixes flutter#193505.

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