Do not apply optimizations based on an element if that element cannot be resolved.

sdk/lib/_internal/compiler/implementation/elements/elements.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.
 
 library elements;
 
 import 'dart:uri';
 
 // TODO(ahe): Rename prefix to 'api' when VM bug is fixed.
 import '../../compiler.dart' as api_e;
@@ skipping 411 matching lines @@
 
   getLibrary() => enclosingElement.getLibrary();
 
   String toString() {
     String n = name.slowToString();
     return '<$n: ${messageKind.message(messageArguments)}>';
   }
 }
 
 /**
- * An ambiguous element represent multiple elements accessible by the same name.
+ * An ambiguous element represents multiple elements accessible by the same
+ * name.
  *
  * Ambiguous elements are created during handling of import/export scopes. If an
  * ambiguous element is encountered during resolution a warning/error should be
  * reported.
  */
 class AmbiguousElement extends Element {
   /**
    * The message to report on resolving this element.
    */
   final MessageKind messageKind;
@@ skipping 1712 matching lines @@
 
   MetadataAnnotation ensureResolved(Compiler compiler) {
     if (resolutionState == STATE_NOT_STARTED) {
       compiler.resolver.resolveMetadataAnnotation(this);
     }
     return this;
   }
 
   String toString() => 'MetadataAnnotation($value, $resolutionState)';
 }