Fix logic for locating dartfmt

pkg/front_end/lib/src/codegen/tools.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';
 
 /**
  * Type of functions used to compute the contents of a set of generated files.
@@ skipping 220 matching lines @@
   File output(String pkgPath) =>
       new File(join(pkgPath, joinAll(posix.split(outputPath))));
 }
 
 /**
  * A utility class for invoking dartfmt.
  */
 class DartFormat {
   static String get _dartfmtPath {
     String binName = Platform.isWindows ? 'dartfmt.bat' : 'dartfmt';
-    return join(dirname(Platform.resolvedExecutable), binName);
+    for (var loc in [binName, join('dart-sdk', 'bin', binName)]) {
+      var candidatePath = join(dirname(Platform.resolvedExecutable), loc);
+      if (new File(candidatePath).existsSync()) {
+        return candidatePath;
+      }
+    }
+    throw new StateError('Could not find dartfmt executable');
   }
 
   static void formatFile(File file) {
     ProcessResult result = Process.runSync(_dartfmtPath, ['-w', file.path]);
     if (result.exitCode != 0) throw result.stderr;
   }
 
   static String formatText(String text) {
     File file = new File(join(Directory.systemTemp.path, 'gen.dart'));
     file.writeAsStringSync(text);
     ProcessResult result = Process.runSync(_dartfmtPath, ['-w', file.path]);
     if (result.exitCode != 0) throw result.stderr;
     return file.readAsStringSync();
   }
 }