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

Fixes https://github.com/flutter/flutter/issues/180862

### Description

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

7e176f8c3f/engine/src/flutter/lib/ui/painting.dart (L5354-L5369)
- Adds test

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we expect every widget to implement].
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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.

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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();