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 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) {