Use dart2js_plaform.dill from the sdk.

tools/testing/dart/compiler_configuration.dart

Index: tools/testing/dart/compiler_configuration.dart
diff --git a/tools/testing/dart/compiler_configuration.dart b/tools/testing/dart/compiler_configuration.dart
index 009fdfe62bef8ddce681a7d92ccbe1571e5485cb..39944343b3f06952e390a6c926ccc542fc64be4e 100644
--- a/tools/testing/dart/compiler_configuration.dart
+++ b/tools/testing/dart/compiler_configuration.dart
@@ -283,6 +283,9 @@ class Dart2xCompilerConfiguration extends CompilerConfiguration {
     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/"