[flutter_tools] use absolute path for shader lib (#116123)

* [flutter_tools] use absolute path for shader lib

* update tests

Author: Jonah Williams

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

@@ -175,8 +175,7 @@ class ShaderCompiler {
       );
     }
 
-    final String shaderLibPath = _fs.path.join(_fs.path.dirname(impellerc.path), 'shader_lib');
-
+    final String shaderLibPath = _fs.path.join(impellerc.parent.absolute.path, 'shader_lib');
     final List<String> cmd = <String>[
       impellerc.path,
       target.target,
@@ -190,6 +189,7 @@ class ShaderCompiler {
       '--include=${input.parent.path}',
       '--include=$shaderLibPath',
     ];
+    _logger.printTrace('shaderc command: $cmd');
     final Process impellercProcess = await _processManager.start(cmd);
     final int code = await impellercProcess.exitCode;
     if (code != 0) {

packages/flutter_tools/test/general.shard/asset_bundle_test.dart

@@ -17,7 +17,7 @@ import 'package:flutter_tools/src/globals.dart' as globals;
 import '../src/common.dart';
 import '../src/context.dart';
 
-const String shaderLibDir = './shader_lib';
+const String shaderLibDir = '/./shader_lib';
 
 void main() {
   group('AssetBundle.build', () {

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

@@ -263,7 +263,7 @@ void main() {
         '--input=/shader.glsl',
         '--input-type=frag',
         '--include=/',
-        '--include=./shader_lib',
+        '--include=/./shader_lib',
       ]),
       FakeCommand(command: <String>[
         'codesign',

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

@@ -16,7 +16,7 @@ import '../../../src/common.dart';
 import '../../../src/fake_process_manager.dart';
 
 const String fragDir = '/shaders';
-const String shaderLibDir = './shader_lib';
+const String shaderLibDir = '/./shader_lib';
 const String fragPath = '/shaders/my_shader.frag';
 const String notFragPath = '/shaders/not_a_frag.file';
 const String outputSpirvPath = '/output/shaders/my_shader.frag.spirv';