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

tests/compiler/dart2js/mock_compiler.dart

Index: tests/compiler/dart2js/mock_compiler.dart
===================================================================
--- tests/compiler/dart2js/mock_compiler.dart	(revision 16853)
+++ tests/compiler/dart2js/mock_compiler.dart	(working copy)
@@ -52,9 +52,11 @@
   class JSArray {
     var length;
     operator[](index) {}
+    var add;
   }
   class JSString {
     var length;
+    operator[](index) {}
   }
   class JSNumber {
   }
@@ -233,6 +235,13 @@
     if (sourceFile == null) throw new ArgumentError(uri);
     return new Script(uri, sourceFile);
   }
+
+  Element lookupElementIn(ScopeContainerElement container, SourceString name) {
+    Element element = container.localLookup(name);
+    return element != null
+        ? element
+        : new ErroneousElement(null, null, name, container);
+  }
 }
 
 void compareWarningKinds(String text, expectedWarnings, foundWarnings) {