Remove MIPS support

tests/standalone/io/test_extension_test.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.
 //
 // Dart test program for testing native extensions.
 
 import "package:expect/expect.dart";
 import "package:path/path.dart";
 import 'dart:async';
 import 'dart:io';
@@ skipping 39 matching lines @@
     default:
       Expect.fail('Unknown operating system ${Platform.operatingSystem}');
   }
 }
 
 String getArchFromBuildDir(String buildDirectory) {
   if (buildDirectory.endsWith('SIMARM')) return '';
   if (buildDirectory.endsWith('SIMARM64')) return '';
   if (buildDirectory.endsWith('SIMDBC')) return '';
   if (buildDirectory.endsWith('SIMDBC64')) return '';
-  if (buildDirectory.endsWith('SIMMIPS')) return '';
   if (buildDirectory.endsWith('ARM')) return '-arm';
   if (buildDirectory.endsWith('ARM64')) return '-arm64';
   if (buildDirectory.endsWith('IA32')) return '-ia32';
-  if (buildDirectory.endsWith('MIPS')) return '-mips';
   if (buildDirectory.endsWith('X64')) return '-x64';
   return 'unknown';
 }
 
 Future testExtension(bool withArchSuffix) {
   String scriptDirectory = dirname(Platform.script.toFilePath());
   String buildDirectory = dirname(Platform.executable);
   Directory tempDirectory =
       Directory.systemTemp.createTempSync('dart_test_extension');
   String testDirectory = tempDirectory.path;
@@ skipping 41 matching lines @@
 }
 
 Future testWithoutArchSuffix() {
   return testExtension(false);
 }
 
 main() async {
   await testWithArchSuffix();
   await testWithoutArchSuffix();
 }