Support terminal colors on Windows 10.

pkg/front_end/lib/src/fasta/colors.dart

Index: pkg/front_end/lib/src/fasta/colors.dart
diff --git a/pkg/front_end/lib/src/fasta/colors.dart b/pkg/front_end/lib/src/fasta/colors.dart
index 6f3214dca91430c06f29dd9fa30b0fdfdc5e53ce..f3891b917d50387f3ccdc2b196cefd395f55e936 100644
--- a/pkg/front_end/lib/src/fasta/colors.dart
+++ b/pkg/front_end/lib/src/fasta/colors.dart
@@ -98,9 +98,9 @@ String white(String string) => wrap(string, WHITE_COLOR);
 /// Note: do not call this method directly, as it is expensive to
 /// compute. Instead, use [CompilerContext.enableColors].
 bool computeEnableColors(CompilerContext context) {
-  if (Platform.isWindows) {
+  if (!Platform.ansiSupported) {
     if (context.options.verbose) {
-      print("Not enabling colors, running on Windows.");
+      print("Not enabling colors, 'Platform.ansiSupported' is false.");
     }
     return false;
   }
@@ -119,6 +119,16 @@ bool computeEnableColors(CompilerContext context) {
     return false;
   }
 
+  if (Platform.isWindows) {
+    if (context.options.verbose) {
+      print("Enabling colors as OS is Windows.");
+    }
+    return true;
+  }
+
+  // We have to check if the terminal actually supports colors. Currently,
+  // `Platform.ansiSupported` is hard-coded to true on non-Windows platforms.

[sortie, 2017/03/16 10:32:03]

This comment may get out of date in the future. You may want to specify when
"currently" is. If Platform.ansiSupported gains support for Unix in the future
(for instance if the vm links in terminfo so there's no need to invoke tput),
the below code should possibly be wholly or partially removed. If this is the
plan, possibly the comment should be of the format "The below code can be
removed when $condition happens."

[ahe, 2017/03/16 11:06:29]

I normally expect that people look in the revision history when they see
comments like this. In the past, I've found that I'm really bad at predicting
exactly what future condition must be satisfied before code can be removed.

+
   // The `-S` option of `tput` allows us to query multiple capabilities at
   // once.
   ProcessResult result = Process.runSync(