Work around Platform.ansiSupported missing from checked-in dart binary.

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

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 // TODO(ahe): Orignially copied from sdk/pkg/compiler/lib/src/colors.dart,
 // merge these two packages.
 library colors;
 
 import 'dart:convert' show JSON;
 
@@ skipping 80 matching lines @@
 String white(String string) => wrap(string, WHITE_COLOR);
 
 /// True if we should enable colors in output. We enable colors when:
 ///  1. stdout and stderr are terminals,
 ///  2. the terminal supports more than 8 colors, and
 ///  3. the terminal's ANSI color codes matches what we expect.
 ///
 /// Note: do not call this method directly, as it is expensive to
 /// compute. Instead, use [CompilerContext.enableColors].
 bool computeEnableColors(CompilerContext context) {
-  if (!Platform.ansiSupported) {
+  bool ansiSupported;

[karlklose, 2017/03/16 13:01:47]

Initialize with 'false'?

[ahe, 2017/03/16 13:11:09]

I have to distinguish between three cases:

* We know that ansi mode isn't supported.

* We know that ansi mode is supported.

* We don't know if ansi mode is supported.

Null represents the latter, and this is used to avoid returning in the case on
line 107.

+  try {
+    ansiSupported = Platform.ansiSupported;
+  } catch (e) {

[karlklose, 2017/03/16 13:01:47]

Could you be more specific about the exception type that you expect when this
getter is not available?

[ahe, 2017/03/16 13:11:10]

Done.

+    // Ignored.

[karlklose, 2017/03/16 13:01:46]

Maybe add something like: 'the getter is not supported in this version of the
VM'.

[ahe, 2017/03/16 13:11:09]

Done.

+  }
+  if (ansiSupported == false) {
     if (context.options.verbose) {
       print("Not enabling colors, 'Platform.ansiSupported' is false.");
     }
     return false;
   }
 
   if (stdioType(stderr) != StdioType.TERMINAL) {
     if (context.options.verbose) {
       print("Not enabling colors, stderr isn't a terminal.");
     }
     return false;
   }
 
   if (stdioType(stdout) != StdioType.TERMINAL) {
     if (context.options.verbose) {
       print("Not enabling colors, stdout isn't a terminal.");
     }
     return false;
   }
 
-  if (Platform.isWindows) {
+  if (ansiSupported == true && 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.
 
   // The `-S` option of `tput` allows us to query multiple capabilities at
@@ skipping 34 matching lines @@
           "${JSON.encode(ALL_CODES)} != ${JSON.encode(allCodes)}.");
     }
     return false;
   }
 
   if (context.options.verbose) {
     print("Enabling colors.");
   }
   return true;
 }