Fixes #180862

Description

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

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

  Lines 5354 to 5369 in 7e176f8

  	static Future<FragmentProgram> fromAsset(String 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;
  	final FragmentProgram? program = _shaderRegistry[encodedKey];
  	if (program != null) {
  	return Future<FragmentProgram>.value(program);
  	}
  	return Future<FragmentProgram>.microtask(() {
  	final program = FragmentProgram._fromAsset(encodedKey);
  	_shaderRegistry[encodedKey] = program;
  	return program;
  	});
  	}

• Adds test

Pre-launch Checklist

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