Prettified timestamps and error reports in --pretty

Timestamps look like Gulp's, with grey times inside white brackets.
Files have cyan filenames, yellow line and column numbers, and grey TS{####} errors. I wonder if those are actually useful for folks using the --pretty CLI: are they used for anything outside Visual Studio... Can we just get rid of them?

Re-uses compiler/program's color logic in compiler/watch. The relevant variables are now exported and marked `@internal`. Is there a preferred way of re-using this code in both those files?

src/compiler/program.ts

@@ -241,22 +241,27 @@ namespace ts {
         return errorMessage;
     }
 
-    const redForegroundEscapeSequence = "\u001b[91m";
-    const yellowForegroundEscapeSequence = "\u001b[93m";
-    const blueForegroundEscapeSequence = "\u001b[93m";
+    /** @internal */
+    export const foregroundColorEscapeSequences = {
+        grey: "\u001b[90m",
+        red: "\u001b[91m",
+        yellow: "\u001b[93m",
+        cyan: "\u001b[96m"
+    };
     const gutterStyleSequence = "\u001b[30;47m";
     const gutterSeparator = " ";
     const resetEscapeSequence = "\u001b[0m";
     const ellipsis = "...";
     function getCategoryFormat(category: DiagnosticCategory): string {
         switch (category) {
-            case DiagnosticCategory.Warning: return yellowForegroundEscapeSequence;
-            case DiagnosticCategory.Error: return redForegroundEscapeSequence;
-            case DiagnosticCategory.Message: return blueForegroundEscapeSequence;
+            case DiagnosticCategory.Warning: return foregroundColorEscapeSequences.yellow;
+            case DiagnosticCategory.Error: return foregroundColorEscapeSequences.red;
+            case DiagnosticCategory.Message: return foregroundColorEscapeSequences.yellow;
         }
     }
 
-    function formatAndReset(text: string, formatStyle: string) {
+    /** @internal */
+    export function formatColorAndReset(text: string, formatStyle: string) {
         return formatStyle + text + resetEscapeSequence;
     }
 
@@ -289,7 +294,7 @@ namespace ts {
                     // If the error spans over 5 lines, we'll only show the first 2 and last 2 lines,
                     // so we'll skip ahead to the second-to-last line.
                     if (hasMoreThanFiveLines && firstLine + 1 < i && i < lastLine - 1) {
-                        context += formatAndReset(padLeft(ellipsis, gutterWidth), gutterStyleSequence) + gutterSeparator + host.getNewLine();
+                        context += formatColorAndReset(padLeft(ellipsis, gutterWidth), gutterStyleSequence) + gutterSeparator + host.getNewLine();
                         i = lastLine - 1;
                     }
 
@@ -300,12 +305,12 @@ namespace ts {
                     lineContent = lineContent.replace("\t", " ");    // convert tabs to single spaces
 
                     // Output the gutter and the actual contents of the line.
-                    context += formatAndReset(padLeft(i + 1 + "", gutterWidth), gutterStyleSequence) + gutterSeparator;
+                    context += formatColorAndReset(padLeft(i + 1 + "", gutterWidth), gutterStyleSequence) + gutterSeparator;
                     context += lineContent + host.getNewLine();
 
                     // Output the gutter and the error span for the line using tildes.
-                    context += formatAndReset(padLeft("", gutterWidth), gutterStyleSequence) + gutterSeparator;
-                    context += redForegroundEscapeSequence;
+                    context += formatColorAndReset(padLeft("", gutterWidth), gutterStyleSequence) + gutterSeparator;
+                    context += foregroundColorEscapeSequences.red;
                     if (i === firstLine) {
                         // If we're on the last line, then limit it to the last character of the last line.
                         // Otherwise, we'll just squiggle the rest of the line, giving 'slice' no end position.
@@ -324,13 +329,19 @@ namespace ts {
                     context += resetEscapeSequence;
                 }
 
-                output += host.getNewLine();
-                output += `${ relativeFileName }(${ firstLine + 1 },${ firstLineChar + 1 }): `;
+                output += formatColorAndReset(relativeFileName, foregroundColorEscapeSequences.cyan);
+                output += "(";
+                output += formatColorAndReset(`${ firstLine + 1 }`, foregroundColorEscapeSequences.yellow);
+                output += ",";
+                output += formatColorAndReset(`${ firstLineChar + 1 }`, foregroundColorEscapeSequences.yellow);
+                output += "): ";
             }
 
             const categoryColor = getCategoryFormat(diagnostic.category);
             const category = DiagnosticCategory[diagnostic.category].toLowerCase();
-            output += `${ formatAndReset(category, categoryColor) } TS${ diagnostic.code }: ${ flattenDiagnosticMessageText(diagnostic.messageText, host.getNewLine()) }`;
+            output += formatColorAndReset(category, categoryColor);
+            output += formatColorAndReset(` TS${ diagnostic.code }: `, foregroundColorEscapeSequences.grey);
+            output += flattenDiagnosticMessageText(diagnostic.messageText, host.getNewLine());
 
             if (diagnostic.file) {
                 output += host.getNewLine();
@@ -339,7 +350,7 @@ namespace ts {
 
             output += host.getNewLine();
         }
-        return output;
+        return output + host.getNewLine();
     }
 
     export function flattenDiagnosticMessageText(messageText: string | DiagnosticMessageChain, newLine: string): string {

src/compiler/watch.ts

@@ -48,6 +48,14 @@ namespace ts {
         };
     }
 
+    export function createWatchDiagnosticReporterWithColor(system = sys): DiagnosticReporter {
+        return diagnostic => {
+            let output = `[${ formatColorAndReset(new Date().toLocaleTimeString(), foregroundColorEscapeSequences.grey) }] `;
+            output += `${flattenDiagnosticMessageText(diagnostic.messageText, system.newLine)}${system.newLine + system.newLine + system.newLine}`;
+            system.write(output);
+        };
+    }
+
     export function reportDiagnostics(diagnostics: Diagnostic[], reportDiagnostic: DiagnosticReporter): void {
         for (const diagnostic of diagnostics) {
             reportDiagnostic(diagnostic);
@@ -131,7 +139,7 @@ namespace ts {
         reportWatchDiagnostic?: DiagnosticReporter
     ): WatchingSystemHost {
         reportDiagnostic = reportDiagnostic || createDiagnosticReporter(system, pretty ? reportDiagnosticWithColorAndContext : reportDiagnosticSimply);
-        reportWatchDiagnostic = reportWatchDiagnostic || createWatchDiagnosticReporter(system);
+        reportWatchDiagnostic = reportWatchDiagnostic || pretty ? createWatchDiagnosticReporterWithColor(system) : createWatchDiagnosticReporter(system);
         parseConfigFile = parseConfigFile || ts.parseConfigFile;
         return {
             system,