Add proper API for creating private symbols wrt a library.

sdk/lib/_internal/lib/mirrors_patch.dart

Index: sdk/lib/_internal/lib/mirrors_patch.dart
diff --git a/sdk/lib/_internal/lib/mirrors_patch.dart b/sdk/lib/_internal/lib/mirrors_patch.dart
index ad0540948519a15d4462f1e6771f60d4528c8a6e..dba0e33d7376ec138d86af3709c4f08766976ccb 100644
--- a/sdk/lib/_internal/lib/mirrors_patch.dart
+++ b/sdk/lib/_internal/lib/mirrors_patch.dart
@@ -8,6 +8,10 @@ import 'dart:_js_mirrors' as js;
 
 patch class MirrorSystem {
   patch static String getName(Symbol symbol) => js.getName(symbol);
+
+  patch static Symbol getSymbol(String name, [LibraryMirror library]) {
+    throw new UnimplementedError("MirrorSystem.getSymbol not implemented");
+  }
 }
 
 patch MirrorSystem currentMirrorSystem() => js.currentJsMirrorSystem;