dart2js: Fix build.

pkg/compiler/lib/src/cps_ir/cps_ir_builder_task.dart

Index: pkg/compiler/lib/src/cps_ir/cps_ir_builder_task.dart
diff --git a/pkg/compiler/lib/src/cps_ir/cps_ir_builder_task.dart b/pkg/compiler/lib/src/cps_ir/cps_ir_builder_task.dart
index 7dbf72c1d23e72af5ba599e6b1dc66c2abd7806d..c791a6c9eefaef542ec8429226a2d2928b04359a 100644
--- a/pkg/compiler/lib/src/cps_ir/cps_ir_builder_task.dart
+++ b/pkg/compiler/lib/src/cps_ir/cps_ir_builder_task.dart
@@ -2783,7 +2783,7 @@ class IrBuilderVisitor extends ast.Visitor<ir.Primitive>
 
     /// Call a helper method from the isolate library. The isolate library uses
     /// its own isolate structure, that encapsulates dart2js's isolate.
-    ir.Primitive buildIsolateHelperInvocation(Element element,
+    ir.Primitive buildIsolateHelperInvocation(MethodElement element,
         CallStructure callStructure) {
       if (element == null) {
         reporter.internalError(node,