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

test/dotnet.Tests/GivenThatDotNetRunsCommands.cs

@@ -0,0 +1,36 @@
+// Copyright (c) .NET Foundation and contributors. All rights reserved. 
+// Licensed under the MIT license. See LICENSE file in the project root for full license information. 
+
+using System.IO;
+using FluentAssertions;
+using Microsoft.DotNet.Tools.Test.Utilities;
+using Xunit;
+
+namespace Microsoft.DotNet.Tests
+{
+    public class GivenThatDotNetRunsCommands : TestBase
+    {
+        [Fact]
+        public void UnresolvedPlatformReferencesFailAsExpected()
+        {
+            var testAssetsManager = GetTestGroupTestAssetsManager("NonRestoredTestProjects");
+            var testInstance = testAssetsManager.CreateTestInstance("TestProjectWithUnresolvedPlatformDependency");
+
+            new RestoreCommand()
+                .WithWorkingDirectory(testInstance.TestRoot)
+                .Execute()
+                .Should()
+                .Fail();
+            new DirectoryInfo(testInstance.TestRoot)
+                .Should()
+                .HaveFile("project.lock.json");
+
+            new DotnetCommand()
+                .ExecuteWithCapturedOutput("crash")
+                .Should()
+                .Fail()
+                .And
+                .HaveStdErrContaining("No executable found matching command \"dotnet-crash\"");
+        }
+    }
+}