Suppress semantic errors in JS only configured projects

(cherry picked from commit 4ee8bdb)

Author: billti

src/server/session.ts

@@ -25,8 +25,14 @@ namespace ts.server {
         return ((1e9 * seconds) + nanoseconds) / 1000000.0;
     }
 
-    function shouldSkipSematicCheck(project: Project) {
-        return (project.projectKind === ProjectKind.Inferred || project.projectKind === ProjectKind.External) && project.isJsOnlyProject();
+    function shouldSkipSemanticCheck(project: Project) {
+        if (project.projectKind === ProjectKind.Inferred || project.projectKind === ProjectKind.External) {
+            return project.isJsOnlyProject();
+        }
+        else {
+            // For configured projects, require that skipLibCheck be set also
+            return project.getCompilerOptions().skipLibCheck && project.isJsOnlyProject();
+        }
     }
 
     interface FileStart {
@@ -447,7 +453,7 @@ namespace ts.server {
         private semanticCheck(file: NormalizedPath, project: Project) {
             try {
                 let diags: Diagnostic[] = [];
-                if (!shouldSkipSematicCheck(project)) {
+                if (!shouldSkipSemanticCheck(project)) {
                     diags = project.getLanguageService().getSemanticDiagnostics(file);
                 }
 
@@ -555,7 +561,7 @@ namespace ts.server {
 
         private getDiagnosticsWorker(args: protocol.FileRequestArgs, isSemantic: boolean, selector: (project: Project, file: string) => Diagnostic[], includeLinePosition: boolean) {
             const { project, file } = this.getFileAndProject(args);
-            if (isSemantic && shouldSkipSematicCheck(project)) {
+            if (isSemantic && shouldSkipSemanticCheck(project)) {
                 return [];
             }
             const scriptInfo = project.getScriptInfoForNormalizedPath(file);