Move main and isolate handling to enqueuer listeners.

pkg/compiler/lib/src/ssa/builder_kernel.dart

Index: pkg/compiler/lib/src/ssa/builder_kernel.dart
diff --git a/pkg/compiler/lib/src/ssa/builder_kernel.dart b/pkg/compiler/lib/src/ssa/builder_kernel.dart
index 0987c17907c5a6838c746c6767deebc687b3c7e5..f182344ac4d62e219d1d6b08ae2ef0ff29427c3d 100644
--- a/pkg/compiler/lib/src/ssa/builder_kernel.dart
+++ b/pkg/compiler/lib/src/ssa/builder_kernel.dart
@@ -199,7 +199,7 @@ class KernelSsaBuilder extends ir.Visitor with GraphBuilder {
       _targetFunction = (target as ir.FunctionDeclaration).function;
       buildFunctionNode(_targetFunction);
     } else {
-      throw 'No case implemented to handle target: $target';
+      throw 'No case implemented to handle target: $target for $targetElement';
     }
     assert(graph.isValid());
     return graph;