[web] Fix loading of fragment shader with space in name. (flutter#180919)

Fixes flutter#180862

### Description

- Adds `assetKey` encoding in `FragmentProgram.fromAsset` in `web_ui` to
be consistent with the implementation in `ui`

https://github.com/flutter/flutter/blob/7e176f8c3fde96342b2c61e5b4043d53113a2b31/engine/src/flutter/lib/ui/painting.dart#L5354-L5369
- Adds test

## Pre-launch Checklist

- [X] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [X] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [X] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [X] I signed the [CLA].
- [X] I listed at least one issue that this PR fixes in the description
above.
- [X] I updated/added relevant documentation (doc comments with `///`).
- [X] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [X] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [X] All existing and new tests are passing.

<!-- Links -->
[Contributor Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#overview
[Tree Hygiene]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md
[test-exempt]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#tests
[Flutter Style Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md
[Features we expect every widget to implement]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md#features-we-expect-every-widget-to-implement
[CLA]: https://cla.developers.google.com/
[flutter/tests]: https://github.com/flutter/tests
[breaking change policy]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#handling-breaking-changes
[Discord]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Chat.md
[Data Driven Fixes]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Data-driven-Fixes.md

Author: ksokolovskyi

engine/src/flutter/lib/web_ui/lib/painting.dart

@@ -1004,7 +1004,12 @@ class ImageDescriptor {
 
 abstract class FragmentProgram {
   static Future<FragmentProgram> fromAsset(String assetKey) {
-    return engine.renderer.createFragmentProgram(assetKey);
+    // The flutter tool converts all asset keys with spaces into URI
+    // encoded paths (replacing ' ' with '%20', for example). We perform
+    // the same encoding here so that users can load assets with the same
+    // key they have written in the pubspec.
+    final String encodedKey = Uri(path: Uri.encodeFull(assetKey)).path;
+    return engine.renderer.createFragmentProgram(encodedKey);
   }
 
   FragmentShader fragmentShader();

engine/src/flutter/lib/web_ui/test/ui/fragment_shader_test.dart

@@ -417,6 +417,12 @@ Future<void> testMain() async {
     }, skip: isWimp); // https://github.com/flutter/flutter/issues/175431
   }
 
+  // Regression test for https://github.com/flutter/flutter/issues/180862.
+  test('fragment shader with space in name loads correctly', () async {
+    assetScope.setAsset('voronoi%20shader', ByteData.sublistView(utf8.encode(kVoronoiShaderSksl)));
+    await expectLater(ui.FragmentProgram.fromAsset('voronoi shader'), completes);
+  }, skip: isWimp); // https://github.com/flutter/flutter/issues/175431
+
   test('getUniformFloat works with correct datatype', () async {
     final ui.FragmentProgram program = await renderer.createFragmentProgram('ink_sparkle');
     final ui.FragmentShader shader = program.fragmentShader();