BindingFlags does not exist in .NET Core netstandard 1.2 class library

In a .NET Core class library project targeting both net452 and netstandard1.3 frameworks, I'm trying to move the latter backwards to netstandard1.2 in order to extend back-compatibility.

Project uses BindingFlags enum and builds fine on net452, as well netstandard1.3, but it fails on netstandard1.2 with a number of errors, all looking either like:

CS0103 The name 'BindingFlags' does not exist in the current context

or like:

CS7069 Reference to type 'BindingFlags' claims it is defined in 'System.Reflection', but it could not be found

Currently, frameworks attribute in project.json is:

"frameworks": {
  "netstandard1.2": {
    "imports": [
      "dnxcore50",
    ],
    "dependencies": {
      "NETStandard.Library": "1.6.0",
      "runtime.any.System.Collections": "4.0.11",
      "System.Collections": "4.0.11",
      "System.Collections.NonGeneric": "4.0.1",
      "System.Console": "4.0.0",
      "System.IO.FileSystem": "4.0.1",
      "System.Reflection.TypeExtensions": "4.1.0",
      "System.Runtime.Serialization.Xml": "4.1.1",
      "System.Text.Encodings.Web": "4.0.0"
    }
  },
  "net452": {
    "frameworkAssemblies": {
      "System.IO": "",
      "System.Runtime.Serialization.Xml": "",
      "System.Xml": "",
      "System.Web": ""
    }
  }
},

and it already lists dependency from System.Reflection.TypeExtensions.

Show potential fixes popup suggests to install "System.Reflection": "4.3.0-preview1-24530-04", only to find that type still shows with the red squiggly line. Another Show potential fixes popup this second time suggests to install "System.Reflection.TypeExtensions": "4.3.0-preview1-24530-04". Of course without affecting the error, that's still there.

Building net452 side goes fine, errors are only on netstandard1.2 build.

What's wrong with this BindingFlags type?