[flutter_tool] Include impellerc output in ShaderCompilerException (#108348)

packages/flutter_tools/lib/src/build_system/targets/shader_compiler.dart

@@ -181,12 +181,16 @@ class ShaderCompiler {
     final Process impellercProcess = await _processManager.start(cmd);
     final int code = await impellercProcess.exitCode;
     if (code != 0) {
-      _logger.printTrace(await utf8.decodeStream(impellercProcess.stdout));
-      _logger.printError(await utf8.decodeStream(impellercProcess.stderr));
+      final String stdout = await utf8.decodeStream(impellercProcess.stdout);
+      final String stderr = await utf8.decodeStream(impellercProcess.stderr);
+      _logger.printTrace(stdout);
+      _logger.printError(stderr);
       if (fatal) {
         throw ShaderCompilerException._(
           'Shader compilation of "${input.path}" to "$outputPath" '
-          'failed with exit code $code.',
+          'failed with exit code $code.\n'
+          'impellerc stdout:\n$stdout\n'
+          'impellerc stderr:\n$stderr',
         );
       }
       return false;

packages/flutter_tools/test/general.shard/build_system/targets/shader_compiler_test.dart

@@ -199,6 +199,8 @@ void main() {
           '--input-type=frag',
           '--include=$fragDir',
         ],
+        stdout: 'impellerc stdout',
+        stderr: 'impellerc stderr',
         exitCode: 1,
       ),
     ]);
@@ -209,14 +211,18 @@ void main() {
       artifacts: artifacts,
     );
 
-    await expectLater(
-      () => shaderCompiler.compileShader(
+    try {
+      await shaderCompiler.compileShader(
         input: fileSystem.file(notFragPath),
         outputPath: outputPath,
         target: ShaderTarget.sksl,
-      ),
-      throwsA(isA<ShaderCompilerException>()),
-    );
+      );
+      fail('unreachable');
+    } on ShaderCompilerException catch (e) {
+      expect(e.toString(), contains('impellerc stdout:\nimpellerc stdout'));
+      expect(e.toString(), contains('impellerc stderr:\nimpellerc stderr'));
+    }
+
     expect(fileSystem.file(outputPath).existsSync(), false);
   });