Use dart2js_plaform.dill from the sdk.

tools/testing/dart/compiler_configuration.dart

 // Copyright (c) 2014, 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 'command.dart';
 import 'configuration.dart';
 import 'path.dart';
 import 'runtime_configuration.dart';
@@ skipping 265 matching lines @@
   static Map<String, List<Uri>> _bootstrapDependenciesCache = {};
 
   Dart2xCompilerConfiguration(this.moniker, Configuration configuration)
       : super._subclass(configuration);
 
   String computeCompilerPath() {
     var prefix = 'sdk/bin';
     var suffix = executableScriptSuffix;
 
     if (_isHostChecked) {
+      if (_useSdk) {
+        throw "--host-checked and --use-sdk cannot be used together";

[Johnni Winther, 2017/07/28 15:54:19]

Why not? Is it short term?

[Siggi Cherem (dart-lang), 2017/07/28 16:45:05]

Added a note in the code.

I actually tried implementing this by defining the DART_VM_OPTIONS environment
and passing the flag, but turns out that --use-sdk runs dart2js from a snapshot,
and sdk snapshots are built without checked mode. If you try to run the
non-checked snapshot and pass `--checked` to the vm, the vm complains.

I believe this is not going to be supported unless we start building checked
snapshots in the sdk in the future.

+      }
       // The script dart2js_developer is not included in the
       // shipped SDK, that is the script is not installed in
       // "$buildDir/dart-sdk/bin/"
       return '$prefix/dart2js_developer$suffix';
     }
 
     if (_useSdk) {
       prefix = '${_configuration.buildDirectory}/dart-sdk/bin';
     }
     return '$prefix/dart2js$suffix';
@@ skipping 445 matching lines @@
   List<String> computeRuntimeArguments(
       RuntimeConfiguration runtimeConfiguration,
       TestInformation info,
       List<String> vmOptions,
       List<String> sharedOptions,
       List<String> originalArguments,
       CommandArtifact artifact) {
     return <String>[];
   }
 }