Issue with PowerUI and 2018.3

I'm having trouble with the PowerUI asset (https://forum.unity.com/threads/powerui-powerful-html-css-ui-framework.196144/) and 2018.3 (project was created under 2018.2). After installing the plugin menu items are supposed to be available but there are build errors following import. Error messages are listed below and it looks to my eye as an incompatibility with some change in the underlying editor. Any help is appreciated:

 Assets\PowerUI\Source\JavaScript\WebAssembly\Emit\ReflectionEmit.cs(380,63): error CS0234: The type or namespace name 'LocalBuilder' does not exist in the namespace 'System.Reflection.Emit' (are you missing an assembly reference?)
 Assets\PowerUI\Source\JavaScript\WebAssembly\Emit\ReflectionEmit.cs(393,33): error CS0234: The type or namespace name 'LocalBuilder' does not exist in the namespace 'System.Reflection.Emit' (are you missing an assembly reference?)
 Assets\PowerUI\Source\JavaScript\WebAssembly\Emit\ReflectionEmit.cs(33,55): error CS0234: The type or namespace name 'Label' does not exist in the namespace 'System.Reflection.Emit' (are you missing an assembly reference?)
 Assets\PowerUI\Source\JavaScript\WebAssembly\Emit\ReflectionEmit.cs(41,33): error CS0234: The type or namespace name 'Label' does not exist in the namespace 'System.Reflection.Emit' (are you missing an assembly reference?)
 Assets\PowerUI\Source\JavaScript\WebAssembly\Emit\ReflectionEmit.cs(62,29): error CS0246: The type or namespace name 'TypeBuilder' could not be found (are you missing a using directive or an assembly reference?)
 Assets\PowerUI\Source\JavaScript\WebAssembly\Emit\ReflectionEmit.cs(56,10): error CS0246: The type or namespace name 'TypeBuilder' could not be found (are you missing a using directive or an assembly reference?)
 Assets\PowerUI\Source\JavaScript\WebAssembly\Emit\ReflectionEmit.cs(328,28): error CS0246: The type or namespace name 'TypeBuilder' could not be found (are you missing a using directive or an assembly reference?)
 Assets\PowerUI\Source\JavaScript\WebAssembly\Emit\ReflectionEmit.cs(433,75): error CS0234: The type or namespace name 'MethodBuilder' does not exist in the namespace 'System.Reflection.Emit' (are you missing an assembly reference?)
 Assets\PowerUI\Source\JavaScript\WebAssembly\Emit\ReflectionEmit.cs(444,75): error CS0234: The type or namespace name 'ConstructorBuilder' does not exist in the namespace 'System.Reflection.Emit' (are you missing an assembly reference?)
 Assets\PowerUI\Source\JavaScript\WebAssembly\Emit\ReflectionEmit.cs(426,34): error CS0234: The type or namespace name 'ILGenerator' does not exist in the namespace 'System.Reflection.Emit' (are you missing an assembly reference?)
 Assets\PowerUI\Source\JavaScript\WebAssembly\Emit\ReflectionEmit.cs(207,33): error CS0234: The type or namespace name 'AssemblyBuilder' does not exist in the namespace 'System.Reflection.Emit' (are you missing an assembly reference?)
 Assets\PowerUI\Source\JavaScript\WebAssembly\Emit\ReflectionEmit.cs(427,34): error CS0234: The type or namespace name 'MethodBuilder' does not exist in the namespace 'System.Reflection.Emit' (are you missing an assembly reference?)
 Assets\PowerUI\Source\JavaScript\WebAssembly\Emit\ReflectionEmit.cs(209,33): error CS0234: The type or namespace name 'ModuleBuilder' does not exist in the namespace 'System.Reflection.Emit' (are you missing an assembly reference?)