Allow multiple-entry libraries in code_transformers

pkg/code_transformers/lib/src/dart_sdk.dart

Index: pkg/code_transformers/lib/src/dart_sdk.dart
diff --git a/pkg/code_transformers/lib/src/dart_sdk.dart b/pkg/code_transformers/lib/src/dart_sdk.dart
index 3b79f368c3cd752457a8a9cc39ad4288006675e8..d31169a8fd8f851539ce4fab7a863d1209d26255 100644
--- a/pkg/code_transformers/lib/src/dart_sdk.dart
+++ b/pkg/code_transformers/lib/src/dart_sdk.dart
@@ -47,3 +47,16 @@ String get dartSdkDirectory {
 
   return null;
 }
+
+/// Variant of [dartSdkDirectory] which includes additional cases only
+/// typically encountered in Dart's testing environment.
+String get testingDartSdkDirectory {
+  var sdkDir = dartSdkDirectory;
+  if (sdkDir == null) {
+    // If we cannot find the SDK dir, then assume this is being run from Dart's
+    // source directory and this script is the main script.
+    sdkDir = path.join(
+        path.dirname(path.fromUri(Platform.script)), '..', '..', '..', 'sdk');
+  }
+  return sdkDir;
+}