Don't send empty environment vars when not Windows PowerShell dev build

This change fixes an issue where $env:PROCESSOR_ARCHITECTURE is not being
set properly in the language server's powershell.exe process which caused
an exception when gathering that variable.  The problem was caused by the
environment variable setting behavior when the Windows PowerShell
developer build setting is set to false.  In this case, an empty
environment variable object was being passed to the process, effectively
clearing its environment variables.

This is one half of the fix to PowerShell/PowerShellEditorServices#346.

Author: daviwil

src/session.ts

@@ -254,12 +254,23 @@ export class SessionManager {
                 "-Command",
                 "& '" + startScriptPath + "' " + startArgs);
 
+            // Set DEVPATH environment variable if necessary
+            if (isWindowsDevBuild) {
+                // The development build looks for this environment variable to
+                // know where to find its assemblies
+                process.env.DEVPATH = path.dirname(powerShellExePath);
+            }
+            else {
+                // It's safe to delete this variable even if it doesn't exist
+                delete process.env.DEVPATH;
+            }
+
             // Launch PowerShell as child process
             this.powerShellProcess =
                 cp.spawn(
                     powerShellExePath,
                     powerShellArgs,
-                    { env: isWindowsDevBuild ? { "DEVPATH": path.dirname(powerShellExePath) } : {} });
+                    { env: process.env });
 
             var decoder = new StringDecoder('utf8');
             this.powerShellProcess.stdout.on(
@@ -545,15 +556,17 @@ export class SessionManager {
     }
 
     private resolvePowerShellPath(powerShellExePath: string): string {
+        var resolvedPath = path.resolve(__dirname, powerShellExePath);
+
         // If the path does not exist, show an error
-        if (!utils.checkIfFileExists(powerShellExePath)) {
+        if (!utils.checkIfFileExists(resolvedPath)) {
             this.setSessionFailure(
-                "powershell.exe cannot be found or is not accessible at path " + powerShellExePath);
+                "powershell.exe cannot be found or is not accessible at path " + resolvedPath);
 
             return null;
         }
 
-        return powerShellExePath;
+        return resolvedPath;
     }
 
     private showSessionMenu() {