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

sdk/lib/_internal/compiler/implementation/compiler.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 dart2js;
 
 /**
  * If true, print a warning for each method that was resolved, but not
  * compiled.
  */
@@ skipping 942 matching lines @@
     unimplemented('Compiler.legDirectory');
   }
 
   // TODO(karlklose): split into findHelperFunction and findHelperClass and
   // add a check that the element has the expected kind.
   Element findHelper(SourceString name)
       => jsHelperLibrary.findLocal(name);
   Element findInterceptor(SourceString name)
       => interceptorsLibrary.findLocal(name);
 
+  Element lookupElementIn(ScopeContainerElement container, SourceString name) {
+    Element element = container.localLookup(name);
+    if (element == null) {
+      throw 'Could not find ${name.slowToString()} in $container';
+    }
+    return element;
+  }
+
   bool get isMockCompilation => false;
 
   Token processAndStripComments(Token currentToken) {
     Token firstToken = currentToken;
     Token prevToken;
     while (currentToken.kind != EOF_TOKEN) {
       if (identical(currentToken.kind, COMMENT_TOKEN)) {
         Token firstCommentToken = currentToken;
         while (identical(currentToken.kind, COMMENT_TOKEN)) {
           currentToken = currentToken.next;
@@ skipping 99 matching lines @@
   // TODO(johnniwinther): Use [spannable] and [message] to provide better
   // information on assertion errors.
   if (condition is Function){
     condition = condition();
   }
   if (spannable == null || !condition) {
     throw new SpannableAssertionFailure(spannable, message);
   }
   return true;
 }