Minor type inference fixes to do with null and bottom types.

pkg/front_end/lib/src/fasta/type_inference/type_schema_environment.dart

Index: pkg/front_end/lib/src/fasta/type_inference/type_schema_environment.dart
diff --git a/pkg/front_end/lib/src/fasta/type_inference/type_schema_environment.dart b/pkg/front_end/lib/src/fasta/type_inference/type_schema_environment.dart
index 3165a2449736cdd067c59477573e858cd0917daf..202293e7b048997012a4c603921dacce00e2008d 100644
--- a/pkg/front_end/lib/src/fasta/type_inference/type_schema_environment.dart
+++ b/pkg/front_end/lib/src/fasta/type_inference/type_schema_environment.dart
@@ -355,6 +355,9 @@ class TypeSchemaEnvironment extends TypeEnvironment {
   bool isBottom(DartType t) {
     if (t is UnknownType) {
       return true;
+    } else if (t is InterfaceType &&
+        identical(t.classNode, coreTypes.nullClass)) {
+      return true;
     } else {
       return super.isBottom(t);
     }