MaybeAdjustFilePath should not collapse multiple slashes

This ends up destroying data that contains double slashes, like URLs
Resolves dotnet#1622

src/Shared/FileUtilities.cs

@@ -413,8 +413,8 @@ internal static string MaybeAdjustFilePath(string value)
                 return value;
             }
 
-            // For Unix-like systems, we may want to convert backslashes to slashes
-            string newValue = Regex.Replace(value, @"[\\/]+", "/");
+            // For Unix-like systems, we want to convert backslashes to slashes
+            string newValue = value.ToSlash();
 
             string quote = string.Empty;
             // Find the part of the name we want to check, that is remove quotes, if present

src/XMakeBuildEngine/UnitTests/Evaluation/EvaluatorFileNormalization_Tests.cs

@@ -0,0 +1,98 @@
+// Copyright (c) Microsoft. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+//-----------------------------------------------------------------------
+// </copyright>
+// <summary>Tests for evaluation</summary>
+//-----------------------------------------------------------------------
+
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Linq;
+using System.Net;
+using System.Threading;
+using System.Xml;
+
+using Microsoft.Build.Collections;
+using Microsoft.Build.Construction;
+using Microsoft.Build.Evaluation;
+using Microsoft.Build.Execution;
+using Microsoft.Build.Framework;
+using Microsoft.Build.Shared;
+using Xunit;
+
+using InvalidProjectFileException = Microsoft.Build.Exceptions.InvalidProjectFileException;
+using ProjectHelpers = Microsoft.Build.UnitTests.BackEnd.ProjectHelpers;
+
+namespace Microsoft.Build.UnitTests.Evaluation
+{
+    /// <summary>
+    /// Tests mainly for how evaluation normalizes input for cross-platform paths
+    /// </summary>
+    public class EvaluatorFileNormalization_Tests : IDisposable
+    {
+        public EvaluatorFileNormalization_Tests()
+        {
+            ProjectCollection.GlobalProjectCollection.UnloadAllProjects();
+            GC.Collect();
+        }
+
+        /// <summary>
+        /// Cleanup
+        /// </summary>
+        public void Dispose()
+        {
+            ProjectCollection.GlobalProjectCollection.UnloadAllProjects();
+            GC.Collect();
+        }
+
+        /// <summary>
+        /// Basic verification -- with TreatAsLocalProperty set, but to a different property than is being passed as a global, the 
+        /// global property overrides the local property.  
+        /// </summary>
+        [Fact]
+        public void MultipleForwardSlashesShouldNotGetCollapsedWhenPathLooksLikeUnixPath()
+        {
+            string content = ObjectModelHelpers.CleanupFileContents(@"
+                             <Project>
+                                    <PropertyGroup>
+                                            <P>/tmp/a//x\\c;ba://</P>
+                                    </PropertyGroup>
+                                    <ItemGroup>
+                                            <I Include=""$(p)""/>
+                                    </ItemGroup>
+
+                                    <Target Name=""Build"">
+                                            <ItemGroup>
+                                                    <T Include=""$(p)""/>
+                                            </ItemGroup>
+                                            <Message Text=""GP[$(GP)]"" Importance=""High""/>
+                                            <Message Text=""P[$(P)]"" Importance=""High""/>
+                                            <Message Text=""I[@(I)]"" Importance=""High""/>
+                                            <Message Text=""T[@(T)]"" Importance=""High""/>
+                                    </Target>
+                            </Project>");
+
+
+            IDictionary<string, string> globalProperties = new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase);
+            globalProperties.Add("GP", @"/tmp/a//x\\c;ba://");
+
+            Project project = new Project(XmlReader.Create(new StringReader(content)), globalProperties, null);
+
+            MockLogger logger = new MockLogger();
+            bool result = project.Build(logger);
+            Assert.Equal(true, result);
+
+            var expectedString = NativeMethodsShared.IsWindows ? @"/tmp/a//x\\c;ba://" : @"/tmp/a//x//c;ba://";
+
+            logger.AssertLogContains($"GP[{expectedString}]");
+            logger.AssertLogContains($"P[{expectedString}]");
+            logger.AssertLogContains($"I[{expectedString}]");
+            logger.AssertLogContains($"T[{expectedString}]");
+
+            Assert.Equal(expectedString, project.GetPropertyValue("GP"));
+            Assert.Equal(expectedString, project.GetPropertyValue("P"));
+            Assert.Equal(expectedString, string.Join(";", project.Items.Select(i => i.EvaluatedInclude)));
+        }
+    }
+}