Merge more Kernel infrastructure from kernel_sdk SDK fork.

sdk/lib/internal/internal.dart

Index: sdk/lib/internal/internal.dart
diff --git a/sdk/lib/internal/internal.dart b/sdk/lib/internal/internal.dart
index 3eda7caf55b4869b81c973dcfa585edd50bae76f..0acf068b50b01a90b95843f6ca01893aeaae40b5 100644
--- a/sdk/lib/internal/internal.dart
+++ b/sdk/lib/internal/internal.dart
@@ -60,3 +60,12 @@ class CodeUnits extends UnmodifiableListBase<int> {
 
   static String stringOf(CodeUnits u) => u._string;
 }
+
+/// Marks a function as an external implementation ("native" in the Dart VM).
+///
+/// Provides a backend-specific String that can be used to identify the
+/// function's implementation.
+class ExternalName {
+  final String name;
+  const ExternalName(this.name);
+}