Use the dart:io Platform.resolvedExecutable API to locate the Dart SDK.

pkg/analyzer/lib/src/util/sdk.dart

+// Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import 'dart:io';
+
+import 'package:path/path.dart' as path;
+
+String getSdkPath([List<String> args]) {
+  // Look for --dart-sdk on the command line.
+  if (args != null) {
+    int index = args.indexOf('--dart-sdk');
+
+    if (index != -1 && (index + 1 < args.length)) {
+      return args[index + 1];
+    }
+
+    for (String arg in args) {
+      if (arg.startsWith('--dart-sdk=')) {
+        return arg.substring('--dart-sdk='.length);
+      }
+    }
+  }
+
+  // Look in env['DART_SDK']
+  if (Platform.environment['DART_SDK'] != null) {
+    return Platform.environment['DART_SDK'];
+  }
+
+  // Use Platform.resolvedExecutable.
+  return path.dirname(path.dirname(Platform.resolvedExecutable));
+}