Add conditional includes in MaterialGraphWindow and MaterialGraphView

com.unity.shadergraph/Editor/Drawing/MaterialGraphEditWindow.cs

 #if UNITY_2018_1
 using GeometryChangedEvent = UnityEngine.Experimental.UIElements.PostLayoutEvent;
 #else
+using UnityEditor.Experimental.UIElements.GraphView;
 using GeometryChangedEvent = UnityEngine.Experimental.UIElements.GeometryChangedEvent;
 #endif
 

com.unity.shadergraph/Editor/Drawing/Views/MaterialGraphView.cs

 using UnityEditor.Graphing;
 using UnityEngine.Experimental.UIElements;
 using Edge = UnityEditor.Experimental.UIElements.GraphView.Edge;
+using Node = UnityEditor.Experimental.UIElements.GraphView.Node;
+#if !UNITY_2018_1
+using UnityEditor.Graphs;
+#endif
 
 namespace UnityEditor.ShaderGraph.Drawing
 {