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

sdk/lib/_internal/compiler/implementation/js_backend/backend.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 js_backend;
 
 typedef void Recompile(Element element);
 
 class ReturnInfo {
   HType returnType;
@@ skipping 763 matching lines @@
       // emitter relies on this list for the order of type checks.
       jsIntClass = compiler.findInterceptor(const SourceString('JSInt')),
       jsDoubleClass = compiler.findInterceptor(const SourceString('JSDouble')),
       jsNumberClass = compiler.findInterceptor(const SourceString('JSNumber')),
       jsNullClass = compiler.findInterceptor(const SourceString('JSNull')),
       jsFunctionClass =
           compiler.findInterceptor(const SourceString('JSFunction')),
       jsBoolClass = compiler.findInterceptor(const SourceString('JSBool'))];
 
     jsArrayClass.ensureResolved(compiler);
-    jsArrayLength =
-        jsArrayClass.lookupLocalMember(const SourceString('length'));
-    jsArrayRemoveLast =
-        jsArrayClass.lookupLocalMember(const SourceString('removeLast'));
-    jsArrayAdd =
-        jsArrayClass.lookupLocalMember(const SourceString('add'));
+    jsArrayLength = compiler.lookupElementIn(
+        jsArrayClass, const SourceString('length'));
+    jsArrayRemoveLast = compiler.lookupElementIn(
+        jsArrayClass, const SourceString('removeLast'));
+    jsArrayAdd = compiler.lookupElementIn(
+        jsArrayClass, const SourceString('add'));
 
     jsStringClass.ensureResolved(compiler);
-    jsStringLength =
-        jsStringClass.lookupLocalMember(const SourceString('length'));
-    jsStringSplit =
-        jsStringClass.lookupLocalMember(const SourceString('split'));
-    jsStringConcat =
-        jsStringClass.lookupLocalMember(const SourceString('concat'));
+    jsStringLength = compiler.lookupElementIn(
+        jsStringClass, const SourceString('length'));
+    jsStringSplit = compiler.lookupElementIn(
+        jsStringClass, const SourceString('split'));
+    jsStringConcat = compiler.lookupElementIn(
+        jsStringClass, const SourceString('concat'));
 
     for (ClassElement cls in classes) {
       if (cls != null) interceptedClasses[cls] = null;
     }
   }
 
   void addInterceptors(ClassElement cls, Enqueuer enqueuer) {
     if (enqueuer.isResolutionQueue) {
       cls.ensureResolved(compiler);
       cls.forEachMember((ClassElement classElement, Element member) {
@@ skipping 331 matching lines @@
     print("Inferred return types:");
     print("----------------------");
     dumpReturnTypes();
     print("");
     print("Inferred field types:");
     print("------------------------");
     fieldTypes.dump();
     print("");
   }
 }