diff --git a/src/harness/harnessLanguageService.ts b/src/harness/harnessLanguageService.ts
index 2a6ed85f9cd..d4fa0d8844c 100644
--- a/src/harness/harnessLanguageService.ts
+++ b/src/harness/harnessLanguageService.ts
@@ -568,6 +568,10 @@ namespace Harness.LanguageService {
             return this.host.getCurrentDirectory();
         }
 
+        getEnvironmentVariable(name: string): string {
+            return ts.sys.getEnvironmentVariable(name);
+        }
+
         readDirectory(path: string, extension?: string): string[] {
             throw new Error("Not implemented Yet.");
         }
@@ -644,4 +648,3 @@ namespace Harness.LanguageService {
         getPreProcessedFileInfo(fileName: string, fileContents: string): ts.PreProcessedFileInfo { throw new Error("getPreProcessedFileInfo is not available using the server interface."); }
     }
 }
- 
\ No newline at end of file
diff --git a/tests/cases/unittests/cachingInServerLSHost.ts b/tests/cases/unittests/cachingInServerLSHost.ts
index 2b6e24e5770..182c86a22e1 100644
--- a/tests/cases/unittests/cachingInServerLSHost.ts
+++ b/tests/cases/unittests/cachingInServerLSHost.ts
@@ -46,6 +46,7 @@ module ts {
             getCurrentDirectory: (): string => {
                 return "";
             },
+            getEnvironmentVariable: (name: string) => "",
             readDirectory: (path: string, extension?: string, exclude?: string[]): string[] => {
                 throw new Error("NYI");
             },
@@ -79,8 +80,8 @@ module ts {
         let projectService = new server.ProjectService(serverHost, logger);
         let rootScriptInfo = projectService.openFile(rootFile, /* openedByClient */true);
         let project = projectService.createInferredProject(rootScriptInfo);
-        project.setProjectOptions( {files: [rootScriptInfo.fileName], compilerOptions: {module: ts.ModuleKind.AMD} } ); 
-        return { 
+        project.setProjectOptions( {files: [rootScriptInfo.fileName], compilerOptions: {module: ts.ModuleKind.AMD} } );
+        return {
             project,
             rootScriptInfo
         };
@@ -97,22 +98,22 @@ module ts {
                 name: "c:/f1.ts",
                 content: `foo()`
             };
-            
+
             let serverHost = createDefaultServerHost({ [root.name]: root, [imported.name]: imported });
             let { project, rootScriptInfo } = createProject(root.name, serverHost);
 
             // ensure that imported file was found
             let diags = project.compilerService.languageService.getSemanticDiagnostics(imported.name);
             assert.equal(diags.length, 1);
-            
+
             let originalFileExists = serverHost.fileExists;
             {
                 // patch fileExists to make sure that disk is not touched
                 serverHost.fileExists = (fileName): boolean => {
-                    assert.isTrue(false, "fileExists should not be called"); 
+                    assert.isTrue(false, "fileExists should not be called");
                     return false;
                 };
-                
+
                 let newContent = `import {x} from "f1"
                 var x: string = 1;`;
                 rootScriptInfo.editContent(0, rootScriptInfo.content.length, newContent);
@@ -133,7 +134,7 @@ module ts {
                 };
                 let newContent = `import {x} from "f2"`;
                 rootScriptInfo.editContent(0, rootScriptInfo.content.length, newContent);
-                
+
                 try {
                     // trigger synchronization to make sure that LSHost will try to find 'f2' module on disk
                     project.compilerService.languageService.getSemanticDiagnostics(imported.name);
@@ -142,7 +143,7 @@ module ts {
                 catch(e) {
                     assert.isTrue(e.message.indexOf(`Could not find file: '${imported.name}'.`) === 0);
                 }
-                
+
                 assert.isTrue(fileExistsIsCalled);
             }
             {
@@ -150,45 +151,45 @@ module ts {
                 serverHost.fileExists = (fileName): boolean => {
                     if (fileName === "lib.d.ts") {
                         return false;
-                    }                    
+                    }
                     fileExistsCalled = true;
                     assert.isTrue(fileName.indexOf('/f1.') !== -1);
                     return originalFileExists(fileName);
                 };
-                
+
                 let newContent = `import {x} from "f1"`;
                 rootScriptInfo.editContent(0, rootScriptInfo.content.length, newContent);
                 project.compilerService.languageService.getSemanticDiagnostics(imported.name);
                 assert.isTrue(fileExistsCalled);
-                
+
                 // setting compiler options discards module resolution cache
                 fileExistsCalled = false;
-                
+
                 let opts = ts.clone(project.projectOptions);
                 opts.compilerOptions = ts.clone(opts.compilerOptions);
                 opts.compilerOptions.target = ts.ScriptTarget.ES5;
                 project.setProjectOptions(opts);
-                
+
                 project.compilerService.languageService.getSemanticDiagnostics(imported.name);
                 assert.isTrue(fileExistsCalled);
             }
         });
-        
+
         it("loads missing files from disk", () => {
             let root: File = {
                 name: 'c:/foo.ts',
                 content: `import {x} from "bar"`
             };
-            
+
             let imported: File = {
                 name: 'c:/bar.d.ts',
                 content: `export var y = 1`
-            };            
-            
+            };
+
             let fileMap: Map<File> = { [root.name]: root };
             let serverHost = createDefaultServerHost(fileMap);
             let originalFileExists = serverHost.fileExists;
-            
+
             let fileExistsCalledForBar = false;
             serverHost.fileExists = fileName => {
                 if (fileName === "lib.d.ts") {
@@ -197,22 +198,22 @@ module ts {
                 if (!fileExistsCalledForBar) {
                     fileExistsCalledForBar = fileName.indexOf("/bar.") !== -1;
                 }
-                
+
                 return originalFileExists(fileName);
             };
-            
+
             let { project, rootScriptInfo } = createProject(root.name, serverHost);
 
             let diags = project.compilerService.languageService.getSemanticDiagnostics(root.name);
             assert.isTrue(fileExistsCalledForBar, "'fileExists' should be called");
             assert.isTrue(diags.length === 1, "one diagnostic expected");
             assert.isTrue(typeof diags[0].messageText === "string" && ((<string>diags[0].messageText).indexOf("Cannot find module") === 0), "should be 'cannot find module' message");
-            
+
             // assert that import will success once file appear on disk
             fileMap[imported.name] = imported;
             fileExistsCalledForBar = false;
             rootScriptInfo.editContent(0, rootScriptInfo.content.length, `import {y} from "bar"`)
-            
+
             diags = project.compilerService.languageService.getSemanticDiagnostics(root.name);
             assert.isTrue(fileExistsCalledForBar, "'fileExists' should be called");
             assert.isTrue(diags.length === 0);
diff --git a/tests/cases/unittests/session.ts b/tests/cases/unittests/session.ts
index 41f4bc599f6..a466f6db726 100644
--- a/tests/cases/unittests/session.ts
+++ b/tests/cases/unittests/session.ts
@@ -17,6 +17,7 @@ namespace ts.server {
         createDirectory(): void {},
         getExecutingFilePath(): string { return void 0; },
         getCurrentDirectory(): string { return void 0; },
+        getEnvironmentVariable(name: string): string { return ""; },
         readDirectory(): string[] { return []; },
         exit(): void {}
     };