Forget about tty detection. It won't work in build tools.

Author: DanielRosenwasser

src/compiler/sys.ts

@@ -6,7 +6,6 @@ namespace ts {
         newLine: string;
         useCaseSensitiveFileNames: boolean;
         write(s: string): void;
-        writesToTty?(): boolean;
         readFile(path: string, encoding?: string): string;
         writeFile(path: string, data: string, writeByteOrderMark?: boolean): void;
         watchFile?(path: string, callback: (path: string, removed?: boolean) => void): FileWatcher;
@@ -379,7 +378,6 @@ namespace ts {
                 write(s: string): void {
                     process.stdout.write(s);
                 },
-                writesToTty: () => _tty.isatty(1),
                 readFile,
                 writeFile,
                 watchFile: (fileName, callback) => {

src/compiler/tsc.ts

@@ -104,13 +104,12 @@ namespace ts {
         sys.write(output);
     }
 
-    const shouldUseColors = !!sys.writesToTty && sys.writesToTty();
-    const redForegroundEscapeSequence = shouldUseColors ? "\u001b[91m" : "";
-    const yellowForegroundEscapeSequence = shouldUseColors ? "\u001b[93m" : "";
-    const blueForegroundEscapeSequence = shouldUseColors ? "\u001b[93m" : "";
-    const gutterStyleSequence = shouldUseColors ? "\u001b[100;30m" : "";
-    const gutterSeparator = shouldUseColors ? " " : " | ";
-    const resetEscapeSequence = shouldUseColors ? "\u001b[0m" : "";
+    const redForegroundEscapeSequence = "\u001b[91m";
+    const yellowForegroundEscapeSequence = "\u001b[93m";
+    const blueForegroundEscapeSequence = "\u001b[93m";
+    const gutterStyleSequence = "\u001b[100;30m";
+    const gutterSeparator = " ";
+    const resetEscapeSequence = "\u001b[0m";
     const elipsis = "...";
     const categoryFormatMap: Map<string> = {
         [DiagnosticCategory.Warning]: yellowForegroundEscapeSequence,