Deal with deferred loading in the VM mirrors.

pkg/compiler/lib/src/mirrors/dart2js_library_mirror.dart

Index: pkg/compiler/lib/src/mirrors/dart2js_library_mirror.dart
diff --git a/pkg/compiler/lib/src/mirrors/dart2js_library_mirror.dart b/pkg/compiler/lib/src/mirrors/dart2js_library_mirror.dart
index 46c2a92653dbbcbf97cb2f565430f51d41ddcd27..fd2504aaaee640b85c45a775131b2f649a476115 100644
--- a/pkg/compiler/lib/src/mirrors/dart2js_library_mirror.dart
+++ b/pkg/compiler/lib/src/mirrors/dart2js_library_mirror.dart
@@ -163,6 +163,11 @@ class Dart2JsLibraryDependencyMirror implements LibraryDependencySourceMirror {
   }
 
   List<InstanceMirror> get metadata => const <InstanceMirror>[];
+
+  /*Future<LibraryMirror>*/ loadLibrary() {
+    throw new UnsupportedError(
+        'LibraryDependencyMirror.loadLibrary unsupported.');
+  }
 }
 
 class Dart2JsCombinatorMirror implements CombinatorSourceMirror {