Propagate bailout messages from the CPS IR backends.

pkg/compiler/lib/src/js_backend/codegen/task.dart

 // Copyright (c) 2014, 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.
 
 /// Generate code using the cps-based IR pipeline.
 library code_generator_task;
 
 import 'glue.dart';
 import 'codegen.dart';
 import 'unsugar.dart';
@@ skipping 90 matching lines @@
 
   cps.FunctionDefinition compileToCpsIR(AstElement element) {
     // TODO(sigurdm): Support these constructs.
     if (element.isNative ||
         element.isField) {
       giveUp('unsupported element kind: ${element.name}:${element.kind}');
     }
 
     cps.FunctionDefinition cpsNode = irBuilderTask.buildNode(element);
     if (cpsNode == null) {
-      giveUp('unable to build cps definition of $element');
+      if (irBuilderTask.bailoutMessage == null) {
+        giveUp('unable to build cps definition of $element');
+      } else {
+        giveUp(irBuilderTask.bailoutMessage);
+      }
     }
     if (element.isInstanceMember && !element.isGenerativeConstructorBody) {
       Selector selector = new Selector.fromElement(cpsNode.element);
       if (glue.isInterceptedSelector(selector)) {
         giveUp('cannot compile methods that need interceptor calling '
             'convention.');
       }
     }
     traceGraph("IR Builder", cpsNode);
     new UnsugarVisitor(glue).rewrite(cpsNode);
@@ skipping 100 matching lines @@
   Iterable<CompilerTask> get tasks {
     // TODO(sigurdm): Make a better list of tasks.
     return <CompilerTask>[irBuilderTask]..addAll(fallbackCompiler.tasks);
   }
 
   js.Node attachPosition(js.Node node, AstElement element) {
     return node.withSourceInformation(
         StartEndSourceInformation.computeSourceInformation(element));
   }
 }