[dart:io] Move Platform.ansiSupported to {Stdin,Stdout}.supportsAnsiEscapes

sdk/lib/io/stdio.dart

Index: sdk/lib/io/stdio.dart
diff --git a/sdk/lib/io/stdio.dart b/sdk/lib/io/stdio.dart
index 29ea48d2a4380dbb9ca1459fb12715eeff26bae4..5df26007a8c18292ebec9f6ae9ce27fb0ee8a376 100644
--- a/sdk/lib/io/stdio.dart
+++ b/sdk/lib/io/stdio.dart
@@ -137,6 +137,15 @@ class Stdin extends _StdStream implements Stream<List<int>> {
   external void set lineMode(bool enabled);
 
   /**
+    * When connected to a terminal, whether ANSI codes are supported.

[Lasse Reichstein Nielsen, 2017/03/17 08:55:29]

This is very vague. 

"ANSI codes" should probably be "ANSI escape codes" or "ANSI escape sequences",
because "ANSI codes" means something else
(https://www.census.gov/geo/reference/ansi.html)
It's still a bad name because it's not actually specified by ANSI, it's just
derived from VT-100 terminal escapes for (among other things) selecting ANSI
colors.

Since there is no formal specification, we can't say which escape codes are
supported, so add a paragraph, e.g.:

  The actual escape sequence support may differ between terminals,
  with some terminals supporting more escape sequences than others,
  and some terminals even differing in behavior for the same escape
  sequence.
  The ANSI color selection is generally supported.

[Lasse Reichstein Nielsen, 2017/03/17 08:55:29]

It doesn't say what happens when it's not connected to a terminal.
Would it be more correct to say:
  Whether connected to a terminal that supports "ANSI" escape sequences.
?

[zra, 2017/03/17 15:38:51]

Done.

[zra, 2017/03/17 15:38:51]

Done.

+    *
+    * This is `true` if the TERM environment variable contains the string
+    * 'xterm', except on Windows where support is detected only in recent
+    * versions of Windows 10.

[Lasse Reichstein Nielsen, 2017/03/17 08:55:28]

This is very specific. Consider just saying:

 Not all terminals are recognized, and not all recognized terminals can
 report whether they support ANSI escape sequences, so this value is a  
 best-effort attempt at detecting the support.

Then you can follow up with the preceding paragraph if you think it's useful:
  Currently, a `TERM` environment variable containing the string `xterm`
  will be taken as evidence that ANSI escape sequences are supported.
  On Windows, only versions of Windows 10 after v.1511 ("TH2")
  will be detected as supporting ANSI escape sequences.

Drop "recent", this comment might still be there in five years.

(Is the Windows comment correct? Is it only the Window32 console or also
Powersheell? Will we not detect that ansi.sys is loaded? Will we not detect
terminals like ConEmu, Cmder or Console2?)

[zra, 2017/03/17 15:38:51]

Done.

Flutter is recommending PowerShell on Windows, so I don't think we should go to
great lengths to support anything else.

[Lasse Reichstein Nielsen, 2017/03/17 16:02:49]

This is not Flutter specific, the comment applies to all platforms that support
dart:io.

[zra, 2017/03/17 17:45:35]

I'm afraid I don't follow you. You asked whether various consoles are supported
by this change on Windows. I responded that developing and validating support
beyond what we have a customer requesting is not in scope right now.

+    */
+  external bool get ansiSupported;

[Lasse Reichstein Nielsen, 2017/03/17 08:55:29]

I'd prefer a name like `supportsAnsiEscapes` (or *maybe* just `supportsAnsi`).
Even `isAnsiSupported` or `hasAnsiSupport` are better than the current, but the
first one is more direct.

[zra, 2017/03/17 15:38:51]

Done.

+
+  /**
    * Synchronously read a byte from stdin. This call will block until a byte is
    * available.
    *
@@ -186,9 +195,19 @@ class Stdout extends _StdFileSink implements IOSink {
    */
   int get terminalLines => _terminalLines(_fd);
 
+  /**
+    * When connected to a terminal, whether ANSI codes are supported.
+    *
+    * This is `true` if the TERM environment variable contains the string
+    * 'xterm', except on Windows where support is detected only in recent
+    * versions of Windows 10.
+    */
+  bool get ansiSupported => _ansiSupported(_fd);
+
   external bool _hasTerminal(int fd);
   external int _terminalColumns(int fd);
   external int _terminalLines(int fd);
+  external static bool _ansiSupported(int fd);
 
   /**
    * Get a non-blocking `IOSink`.