Propagate bailout messages from the CPS IR backends.

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

Index: pkg/compiler/lib/src/js_backend/codegen/task.dart
diff --git a/pkg/compiler/lib/src/js_backend/codegen/task.dart b/pkg/compiler/lib/src/js_backend/codegen/task.dart
index d631f286655f710e1b4320367c062ff39a75fd0f..2bd1b14382fe5aeb0948e29f03fe71849f0df732 100644
--- a/pkg/compiler/lib/src/js_backend/codegen/task.dart
+++ b/pkg/compiler/lib/src/js_backend/codegen/task.dart
@@ -108,7 +108,11 @@ class CpsFunctionCompiler implements FunctionCompiler {
 
     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);