Fix subtyping check on number

pkg/dev_compiler/lib/src/compiler/code_generator.dart

Index: pkg/dev_compiler/lib/src/compiler/code_generator.dart
diff --git a/pkg/dev_compiler/lib/src/compiler/code_generator.dart b/pkg/dev_compiler/lib/src/compiler/code_generator.dart
index afabcf3a6cb464103b5cce1d8cd97a446732f367..56daf6b806275feb6cecffa1d04c9c9c330554c1 100644
--- a/pkg/dev_compiler/lib/src/compiler/code_generator.dart
+++ b/pkg/dev_compiler/lib/src/compiler/code_generator.dart
@@ -5624,7 +5624,9 @@ class CodeGenerator extends GeneralizingAstVisitor
   ///
   /// JSNumber is the type that actually "implements" all numbers, hence it's
   /// a subtype of int and double (and num). It's in our "dart:_interceptors".
-  bool _isNumberInJS(DartType t) => rules.isSubtypeOf(t, types.numType);
+  bool _isNumberInJS(DartType t) =>
+      rules.isSubtypeOf(t, types.numType) &&
+      !rules.isSubtypeOf(t, types.nullType);
 
   /// Return true if this is one of the methods/properties on all Dart Objects
   /// (toString, hashCode, noSuchMethod, runtimeType).