Move assertion to realpath evaluation

src/compiler/checker.ts

@@ -25004,7 +25004,6 @@ namespace ts {
                         return;
                     }
                     const file = host.getSourceFile(resolvedDirective.resolvedFileName);
-                    Debug.assert(!!file, `Resolved filename ${resolvedDirective.resolvedFileName} did not map to existing source file. Consider enabling --preserveSymlinks if appropriate`); // tslint:disable-line
                     fileToDirective.set(file.path, key);
                 });
             }

src/compiler/moduleNameResolver.ts

@@ -782,6 +782,7 @@ namespace ts {
         if (traceEnabled) {
             trace(host, Diagnostics.Resolving_real_path_for_0_result_1, path, real);
         }
+        Debug.assert(host.fileExists(real), `${path} linked to non-existing file ${real}`); // tslint:disable-line
         return real;
     }