dart2js: Do not use JSDouble for native behavior return types.

pkg/compiler/lib/src/inferrer/simple_types_inferrer.dart

Index: pkg/compiler/lib/src/inferrer/simple_types_inferrer.dart
diff --git a/pkg/compiler/lib/src/inferrer/simple_types_inferrer.dart b/pkg/compiler/lib/src/inferrer/simple_types_inferrer.dart
index b242c98e140d4a3a58feb1fb02e2585b75769ba2..d394bb7c3dbba41ebd3944c6d0d3442792df9e12 100644
--- a/pkg/compiler/lib/src/inferrer/simple_types_inferrer.dart
+++ b/pkg/compiler/lib/src/inferrer/simple_types_inferrer.dart
@@ -373,9 +373,11 @@ abstract class InferrerEngine<T, V extends TypeSystem>
         mappedType = types.stringType;
       } else if (type.element == compiler.intClass) {
         mappedType = types.intType;
-      } else if (type.element == compiler.doubleClass) {
-        mappedType = types.doubleType;
-      } else if (type.element == compiler.numClass) {
+      } else if (type.element == compiler.numClass ||
+                 type.element == compiler.doubleClass) {
+        // Note: the backend double class is specifically for non-integer
+        // doubles, and a native behavior returning 'double' does not guarantee
+        // a non-integer return type, so we return the number type for those.
         mappedType = types.numType;
       } else if (type.element == compiler.boolClass) {
         mappedType = types.boolType;