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

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

Index: sdk/lib/_internal/compiler/implementation/compiler.dart
===================================================================
--- sdk/lib/_internal/compiler/implementation/compiler.dart	(revision 16853)
+++ sdk/lib/_internal/compiler/implementation/compiler.dart	(working copy)
@@ -960,6 +960,14 @@
   Element findInterceptor(SourceString name)
       => interceptorsLibrary.findLocal(name);
 
+  Element findInScopeContainer(ScopeContainerElement scope, SourceString name) {

[ahe, 2013/01/09 15:46:27]

I really don't like the term "scope container", and you're definitely not
looking up in a scope.

So I'd prefer a different name for the method and the first argument.

Also, this method is a "lookup" not a "find" as it reports an error if the
element isn't found.

[ngeoffray, 2013/01/10 09:41:48]

Renamed to findElementIn.

+    Element element = scope.localLookup(name);
+    if (element == null) {
+      internalError('Could not find ${name.slowToString()} in $scope');

[ahe, 2013/01/09 15:46:27]

Throw an exception instead?

[ngeoffray, 2013/01/10 09:41:48]

Done.

+    }
+    return element;
+  }
+
   bool get isMockCompilation => false;
 
   Token processAndStripComments(Token currentToken) {