Add regression tests.

lib/custom_compiler.dart

Index: lib/custom_compiler.dart
diff --git a/lib/custom_compiler.dart b/lib/custom_compiler.dart
index 122401029741306430dd1ac7a27ff60e47021352..9cf03b9e0e4e5cb250f52a08033ab3c73172f09b 100644
--- a/lib/custom_compiler.dart
+++ b/lib/custom_compiler.dart
@@ -130,6 +130,7 @@ class CustomCompiler extends CompilerImpl {
       handler.isAborting = false;
     }
     enqueuer.resolution.deferredTaskQueue.clear();
+    (libraryLoader as dynamic).currentHandler = null;
   }
 
   bool get isInternalStateConsistent {
@@ -151,6 +152,10 @@ class CustomCompiler extends CompilerImpl {
       print("Resolution deferredTaskQueue isn't empty");
       return false;
     }
+    if ((libraryLoader as dynamic).currentHandler != null) {
+      print("compiler.libraryLoader.currentHandler isn't null");
+      return false;
+    }
     return true;
   }
 }