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 47009848c8972607cc5cc8db586f282668da300d..0a1f74b8f5ad51352a208762ba1cd7b7db6c89e3 100644
--- a/sdk/lib/_internal/lib/mirrors_patch.dart
+++ b/sdk/lib/_internal/lib/mirrors_patch.dart
@@ -8,6 +8,9 @@ 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 lib]) {
+    throw new UnimplementedError("MirrorSystem.getSymbol not implemented");
+  }
 }
 
 patch MirrorSystem currentMirrorSystem() => js.currentJsMirrorSystem;