Speed up tests by not type-checking lib.d.ts unless the test asks for that.

src/compiler/program.ts

@@ -391,6 +391,7 @@ module ts {
                         processImportedModules(file, basePath);
                     }
                     if (isDefaultLib) {
+                        file.isDefaultLib = true;
                         files.unshift(file);
                     }
                     else {