Allow creation of symlink to non-existent target

This action is completely valid on both Windows and Linux (and OS X)
operating systems; tested with `mklink` and `ln -s` respectively.

Note that targets for hard links must exist, thus we check specifically
for symbolic links.

Both the path globber in session state and the `New-Item` implementation
needed to be fixed to allow the target not to exist.

Resolves #801.

Also enable symbolic link tests on Windows.

src/System.Management.Automation/engine/SessionStateContainer.cs

@@ -3712,6 +3712,8 @@ namespace System.Management.Automation
                     }
 
                     bool isSymbolicJunctionOrHardLink = false;
+                    // Symbolic link targets are allowed to not exist on both Windows and Linux
+                    bool allowNonexistingPath = false;
                     
                     if(type != null)
                     {
@@ -3720,6 +3722,7 @@ namespace System.Management.Automation
                         if (typeEvaluator.IsMatch("symboliclink") || typeEvaluator.IsMatch("junction") || typeEvaluator.IsMatch("hardlink"))
                         {                            
                             isSymbolicJunctionOrHardLink = true;
+                            allowNonexistingPath = typeEvaluator.IsMatch("symboliclink");
                         }
                     }
 
@@ -3742,7 +3745,7 @@ namespace System.Management.Automation
 
                         var globbedTarget = Globber.GetGlobbedProviderPathsFromMonadPath(
                             targetPath,
-                            false,
+                            allowNonexistingPath,
                             context,
                             out targetProvider,
                             out targetProviderInstance);

src/System.Management.Automation/namespaces/FileSystemProvider.cs

@@ -2125,9 +2125,14 @@ namespace Microsoft.PowerShell.Commands
 
                     bool exists = false;
 
+                    // It is legal to create symbolic links to non-existing targets on
+                    // both Windows and Linux. It is not legal to create hard links to
+                    // non-existing targets on either Windows or Linux.
                     try
                     {
-                        exists = CheckItemExists(strTargetPath, out isDirectory);
+                        exists = (itemType == ItemType.SymbolicLink)
+                            ? true // pretend it exists if we're making a symbolic link
+                            : CheckItemExists(strTargetPath, out isDirectory);
                     }
                     catch (Exception e)
                     {

test/powershell/New-Item.Tests.ps1

@@ -104,7 +104,7 @@ Describe "New-Item" {
 	Test-Path $FullyQualifiedFile | Should Be $false
     }
 
-    It "Should create a symbolic link of a file without error" -Skip:$IsWindows {
+    It "Should create a symbolic link of a file without error" {
 	New-Item -Name $testfile -Path $tmpDirectory -ItemType file
 	Test-Path $FullyQualifiedFile | Should Be $true
 
@@ -116,7 +116,18 @@ Describe "New-Item" {
         $fileInfo.LinkType | Should Be "SymbolicLink"
     }
 
-    It "Should create a symbolic link from directory without error" -Skip:$IsWindows {
+    It "Should create a symbolic link to a non-existing file without error" {
+        $target = Join-Path $tmpDirectory "totallyBogusFile"
+        New-Item -ItemType SymbolicLink -Target $target -Name $testlink -Path $tmpDirectory
+        Test-Path $FullyQualifiedLink | Should Be $true
+
+        $fileInfo = Get-ChildItem $FullyQualifiedLink
+        $fileInfo.Target | Should Be $target
+        Test-Path $fileInfo.Target | Should be $false
+        $fileInfo.LinkType | Should Be "SymbolicLink"
+    }
+
+    It "Should create a symbolic link from directory without error" {
 	New-Item -Name $testFolder -Path $tmpDirectory -ItemType directory
 	Test-Path $FullyQualifiedFolder | Should Be $true