Better record compile-time errors on elements

pkg/compiler/lib/src/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;
 
 const VERBOSE_OPTIMIZER_HINTS = false;
 
 class JavaScriptItemCompilationContext extends ItemCompilationContext {
   final Set<HInstruction> boundsChecked = new Set<HInstruction>();
@@ skipping 1549 matching lines @@
     }
     for (BackendImpact otherImpact in impact.otherImpacts) {
       enqueueImpact(enqueuer, otherImpact, registry);
     }
   }
 
   WorldImpact codegen(CodegenWorkItem work) {
     Element element = work.element;
     if (compiler.elementHasCompileTimeError(element)) {
       DiagnosticMessage message =
-          compiler.elementsWithCompileTimeErrors[element];
+          compiler.elementsWithCompileTimeErrors[element].last;
       String messageText = message.message.computeMessage();
       jsAst.LiteralString messageLiteral =
           js.escapedString("Compile time error in $element: $messageText");
       generatedCode[element] = js(
           "function () { throw new Error(#); }", [messageLiteral]);
       return const CodegenImpact();
     }
     var kind = element.kind;
     if (kind == ElementKind.TYPEDEF) {
       return const WorldImpact();
@@ skipping 1498 matching lines @@
   }
 }
 
 /// Records that [constant] is used by the element behind [registry].
 class Dependency {
   final ConstantValue constant;
   final Element annotatedElement;
 
   const Dependency(this.constant, this.annotatedElement);
 }