Remove nasty ANSI workaround for Windows (#8839)

The problem has been fixed upstream in the Dart VM.

This simplifies our setup instructions on Windows (will update the wiki).
Furthermore, this also means that going forward there is no diffrence between PowerShell and Cmd for the Flutter experience on Windows.

bin/flutter.ps1

-# Copyright 2017 The Chromium Authors. All rights reserved.
-# Use of this source code is governed by a BSD-style license that can be
-# found in the LICENSE file.
-
-# This is a wrapper arround flutter.bat, which checks if the terminal supports
-# ANSI codes to work around https://github.com/dart-lang/sdk/issues/28614.
-
-$ErrorActionPreference = "Stop"
-
-Add-Type -MemberDefinition @"
-[DllImport("kernel32.dll", SetLastError=true)]
-public static extern bool SetConsoleMode(IntPtr hConsoleHandle, int mode);
-[DllImport("kernel32.dll", SetLastError=true)]
-public static extern IntPtr GetStdHandle(int handle);
-[DllImport("kernel32.dll", SetLastError=true)]
-public static extern bool GetConsoleMode(IntPtr handle, out int mode);
-"@ -Namespace Win32 -Name NativeMethods
-
-If (Test-Path Env:\FLUTTER_ANSI_TERMINAL) { Remove-Item Env:\FLUTTER_ANSI_TERMINAL }
-$stdout = [Win32.NativeMethods]::GetStdHandle(-11) # STD_OUTPUT_HANDLE
-If ($stdout -ne -1) {
-    $mode = 0
-    If ([Win32.NativeMethods]::GetConsoleMode($stdout, [ref]$mode)) {
-        $mode = $mode -bor 4 # ENABLE_VIRTUAL_TERMINAL_PROCESSING
-        If ([Win32.NativeMethods]::SetConsoleMode($stdout, $mode)) {
-            $env:FLUTTER_ANSI_TERMINAL = "true"
-        }
-    }
-}
-
-# Cannot use $PSScriptRoot as that requires PowerShell 3 or newer
-$scriptRoot = Split-Path $MyInvocation.MyCommand.Path -Parent
-& $scriptRoot\flutter.bat $args
-exit $LastExitCode

packages/flutter_tools/lib/src/base/logger.dart

@@ -249,10 +249,12 @@ enum _LogType {
 
 class AnsiTerminal {
   AnsiTerminal() {
-    final String term = platform.environment['TERM'];
-    // FLUTTER_ANSI_TERMINAL is a work-around for https://github.com/dart-lang/sdk/issues/28614
-    final bool flutterAnsiTerm = platform.environment.containsKey('FLUTTER_ANSI_TERMINAL');
-    supportsColor = (term != null && term != 'dumb') || flutterAnsiTerm;
+    if (platform.isWindows) {
+      supportsColor = platform.ansiSupported;
+    } else {
+      final String term = platform.environment['TERM'];
+      supportsColor = (term != null && term != 'dumb');
+    }
   }
 
   static const String _bold  = '\u001B[1m';