Add missing 'throw' to ImageFilter.shader's validation check (#180923)

An error StringBuffer was created and populated, but was left unused.
This PR `throw`s it.

This fixes and re-enables fragment_shader_test.dart's ImageFilter error
test. The test is re-enabled as an impeller-only test because
ImageFilter.shader requires impeller.

Fixes #180763

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

@@ -4400,6 +4400,7 @@ abstract class ImageFilter {
       if (invalidSampler) {
         buffer.write('The shader is missing a sampler uniform.\n');
       }
+      throw StateError(buffer.toString());
     }
     return _FragmentShaderImageFilter(shader);
   }

engine/src/flutter/testing/dart/fragment_shader_test.dart

@@ -489,39 +489,36 @@ void main() async {
     shader.dispose();
   });
 
-  test('ImageFilter.shader errors if shader does not have correct uniform layout', () async {
-    // TODO(gaaclarke): This test was disabled for a long time and has been
-    // atrophied. Fix it or remove it.
-    print('Atrophied test is disabled.');
-    return;
-    // ignore: dead_code
-    const shaders = <String>[
-      'no_uniforms.frag.iplr',
-      'missing_size.frag.iplr',
-      'missing_texture.frag.iplr',
-    ];
-    const errors = <(bool, bool)>[(true, true), (true, false), (false, false)];
-    for (var i = 0; i < 3; i++) {
-      final String fileName = shaders[i];
-      final FragmentProgram program = await FragmentProgram.fromAsset(fileName);
-      final FragmentShader shader = program.fragmentShader();
+  _runImpellerTest(
+    'ImageFilter.shader errors if shader does not have correct uniform layout',
+    () async {
+      const List<({String file, bool floatError, bool samplerError})> testCases = [
+        (file: 'no_uniforms.frag.iplr', floatError: true, samplerError: true),
+        (file: 'missing_size.frag.iplr', floatError: true, samplerError: false),
+        (file: 'missing_texture.frag.iplr', floatError: false, samplerError: true),
+      ];
 
-      Object? error;
-      try {
-        ImageFilter.shader(shader);
-      } catch (err) {
-        error = err;
+      for (final testCase in testCases) {
+        final FragmentProgram program = await FragmentProgram.fromAsset(testCase.file);
+        final FragmentShader shader = program.fragmentShader();
+
+        Object? error;
+        try {
+          ImageFilter.shader(shader);
+        } catch (err) {
+          error = err;
+        }
+        expect(error, isA<StateError>());
+        final errorMessage = error.toString();
+        if (testCase.floatError) {
+          expect(errorMessage, contains('shader has fewer than two float'));
+        }
+        if (testCase.samplerError) {
+          expect(errorMessage, contains('shader is missing a sampler uniform'));
+        }
       }
-      expect(error is StateError, true);
-      final (bool floatError, bool samplerError) = errors[i];
-      if (floatError) {
-        expect(error.toString(), contains('shader has fewer than two float'));
-      }
-      if (samplerError) {
-        expect(error.toString(), contains('shader is missing a sampler uniform'));
-      }
-    }
-  });
+    },
+  );
 
   _runImpellerTest('Shader Compiler appropriately pads vec3 uniform arrays', () async {
     // TODO(gaaclarke): This test was disabled for a long time and has been