Throw when reflecting on elements not covered by a `MirrorsUsed` annotation.

sdk/lib/_internal/compiler/implementation/mirrors_used.dart

Index: sdk/lib/_internal/compiler/implementation/mirrors_used.dart
diff --git a/sdk/lib/_internal/compiler/implementation/mirrors_used.dart b/sdk/lib/_internal/compiler/implementation/mirrors_used.dart
index 9c8df9cab5fbe0355c21246f47bedc7b714f2344..e85b4e7d243258e7c9a07b6e58ded1cd223c2004 100644
--- a/sdk/lib/_internal/compiler/implementation/mirrors_used.dart
+++ b/sdk/lib/_internal/compiler/implementation/mirrors_used.dart
@@ -559,6 +559,10 @@ class MirrorUsageBuilder {
   Element findLocalMemberIn(Element element, SourceString name) {
     if (element is ScopeContainerElement) {
       ScopeContainerElement scope = element;
+      if (element.isClass()) {
+        ClassElement cls = element;
+        cls.ensureResolved(compiler);
+      }
       return scope.localLookup(name);
     }
     return null;