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

sdk/lib/_internal/compiler/implementation/js_backend/backend.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of js_backend;
 
 class JavaScriptItemCompilationContext extends ItemCompilationContext {
   final Set<HInstruction> boundsChecked;
 
   JavaScriptItemCompilationContext()
@@ skipping 1537 matching lines @@
           targetsUsed.add(field.getter);
           targetsUsed.add(field.setter);
         } else {
           targetsUsed.add(target);
         }
       }
     }
     if (metaTargets != null) metaTargetsUsed.addAll(metaTargets);
   }
 
+  /**
+   * Returns `true` if [element] can be accessed through reflection, that is,
+   * is in the set of elements covered by a `MirrorsUsed` annotation.
+   *
+   * This property is used to tag emitted elements with a marker which is
+   * checked by the runtime system to throw an exception if an element is
+   * accessed (invoked, get, set) that is not accessible for the reflective
+   * system.
+   */
+  bool isAccessibleByReflection(Element element) {
+    if (hasInsufficientMirrorsUsed) return true;
+    return isNeededForReflection(element);
+  }
+
+  /**
+   * Returns `true` if the emitter must emit the element even though there
+   * is no direct use in the program, but because the reflective system may
+   * need to access it.
+   */
   bool isNeededForReflection(Element element) {
     if (hasInsufficientMirrorsUsed) return isTreeShakingDisabled;
     /// Record the name of [element] in [symbolsUsed]. Return true for
     /// convenience.
     bool registerNameOf(Element element) {
       symbolsUsed.add(element.name.slowToString());
       if (element.isConstructor()) {
         symbolsUsed.add(element.getEnclosingClass().name.slowToString());
       }
       return true;
@@ skipping 74 matching lines @@
     copy(constant.values);
     copy(constant.protoValue);
     copy(constant);
   }
 
   void visitConstructed(ConstructedConstant constant) {
     copy(constant.fields);
     copy(constant);
   }
 }