dotnet crashes with an unresolved 'platform' dependency.

Fixed by calling TryGetValue instead of a dictionary indexer when looking up platform dependencies in ProjectContextBuilder.

Fix #2693

TestAssets/NonRestoredTestProjects/TestProjectWithUnresolvedPlatformDependency/Program.cs

@@ -0,0 +1,12 @@
+using System;
+
+namespace ConsoleApplication
+{
+    public class Program
+    {
+        public static void Main(string[] args)
+        {
+            Console.WriteLine("Hello World!");
+        }
+    }
+}