Update incremental load example so you can point to different sdk

pkg/front_end/example/incremental_reload/compiler_with_invalidation.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.
 
 /// A wrapper on top of the [IncrementalKernelGenerator] that tracks
 /// file modifications between subsequent compilation requests and only
 /// invalidates those files that appear to be modified.
 library front_end.example.incremental_reload.compiler_with_invalidation;
 
 import 'dart:io';
 import 'dart:async';
 
 import 'package:front_end/compiler_options.dart';
 import 'package:front_end/incremental_kernel_generator.dart';
 import 'package:front_end/src/byte_store/file_byte_store.dart';
 import 'package:front_end/src/byte_store/byte_store.dart';
 import 'package:kernel/ast.dart';
 import 'package:kernel/binary/limited_ast_to_binary.dart';
 
 /// Create an instance of an [IncrementalCompiler] to compile a program whose
 /// main entry point file is [entry]. This uses some default options
 /// for the location of the sdk and temporary folder to save intermediate
 /// results.
 // TODO(sigmund): make this example work outside of the SDK repo.
 Future<IncrementalCompiler> createIncrementalCompiler(String entry,
-    {bool persistent: true}) {
+    {bool persistent: true, Uri argSdkRoot}) {

[Siggi Cherem (dart-lang), 2017/08/15 00:37:14]

nit: rename to sdkRoot, possibly remove the TODO :) above?

[aam, 2017/08/15 03:59:49]

Done.

   var entryUri = Uri.base.resolve(entry);
   var dartVm = Uri.base.resolve(Platform.resolvedExecutable);
-  var sdkRoot = dartVm.resolve("patched_sdk/");
+  var sdkRoot = argSdkRoot ?? dartVm.resolve("patched_sdk/");
   var tmpDir = Directory.systemTemp.createTempSync('ikg_cache');
   var options = new CompilerOptions()
     ..sdkRoot = sdkRoot
     ..packagesFileUri = Uri.base.resolve('.packages')
     ..strongMode = false
     // Note: we do not report error on the console because the incremental
     // compiler is an ongoing background service that shouldn't polute stdout.
     // TODO(sigmund): do something with the errors.
     ..onError = (_) {}
     ..byteStore =
@@ skipping 123 matching lines @@
     result.errorSeen = true;
   }
 
   result.changed = compiler.changed;
   result.totalFiles = compiler.lastModified.length;
   result.invalidateTime = compiler.invalidateTime;
   result.compileTime = compiler.compileTime;
   result.reloadTime = 0;
   return result;
 }