- Use `Uri.parse()` to extract pathname.
- Remove unused code from `utils.dart`.
- Add test for URL encoding.

(need to wait for https://github.com/flutter/flutter/pull/126851 to make it into Google3)

To avoid confusion and unpredictable errors (e.g. https://github.com/flutter/flutter/issues/127462), I'm adding this assert to disallow this pattern.

Fixes https://github.com/flutter/flutter/issues/127462

Remove the following APIs and export them directly from `dart:ui_web`:
- `urlStrategy` getter and setter
- `HashUrlStrategy`
- `PlatformLocation` and `BrowserPlatformLocation` (keep the façades for non-web platforms)

Depends on https://github.com/flutter/engine/pull/42043
Depends on https://github.com/flutter/engine/pull/42252

Part of https://github.com/flutter/flutter/issues/126831

This PR fixes the dropping of both the `path` and `search` fields from the platform location in the URL when using Flutter Web and brings it in par with similar technologies (e.g. React Router).
It allows developers to keep the original path and/or search parameters in the URL, which are perfectly valid even while fragment routing is present.

**Example use case:**
Call a Flutter Web app with initial config parameters in the URL: 
`http://my-flutter.app/?skipIntro=true`

**Example:**
Before initial routing:
`http://localhost:45389/?foo=bar`
After routing:
`http://localhost:45389/#/menu`
After routing (with fix):
`http://localhost:45389/?foo=bar#menu`

Fixes #116415

This is based off (and dependent on) these changes in the web engine that expose web-specific APIs: https://github.com/flutter/engine/pull/40608