Fix subtyping check on number

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

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import 'dart:collection' show HashMap, HashSet;
 import 'dart:math' show min, max;
 
 import 'package:analyzer/analyzer.dart' hide ConstantEvaluator;
 import 'package:analyzer/dart/ast/ast.dart';
@@ skipping 5606 matching lines @@
     }
     return node..sourceInformation = original;
   }
 
   /// Returns true if this is any kind of object represented by `Number` in JS.
   ///
   /// In practice, this is 4 types: num, int, double, and JSNumber.
   ///
   /// 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).
   ///
   /// Operator == is excluded, as it is handled as part of the equality binary
   /// operator.
   bool isObjectMember(String name) {
     // We could look these up on Object, but we have hard coded runtime helpers
     // so it's not really providing any benefit.
     switch (name) {
@@ skipping 193 matching lines @@
   if (targetIdentifier.staticElement is! PrefixElement) return false;
   var prefix = targetIdentifier.staticElement as PrefixElement;
 
   // The library the prefix is referring to must come from a deferred import.
   var containingLibrary = resolutionMap
       .elementDeclaredByCompilationUnit(target.root as CompilationUnit)
       .library;
   var imports = containingLibrary.getImportsWithPrefix(prefix);
   return imports.length == 1 && imports[0].isDeferred;
 }