Make the Platform.environment return value an unmodifiable map

sdk/lib/io/platform_impl.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.
 
 part of dart.io;
 
 class _Platform {
   external static int _numberOfProcessors();
   external static String _pathSeparator();
   external static String _operatingSystem();
   external static _localHostname();
   external static _executable();
   external static _environment();
   external static List<String> _executableArguments();
   external static String _packageRoot();
   external static String _version();
 
+  // Cache the OS environemnt. This can be an OSError instance if
+  // retrieving the environment failed.
+  static var _environmentCache;
+
   static int get numberOfProcessors => _numberOfProcessors();
   static String get pathSeparator => _pathSeparator();
   static String get operatingSystem => _operatingSystem();
   static Uri script = _script();
   static Uri _script() {
     // The embedder (Dart executable) creates the Platform._nativeScript field.
     var s = Platform._nativeScript;
     if (s.startsWith('http:') ||
         s.startsWith('https:') ||
         s.startsWith('file:')) {
@@ skipping 10 matching lines @@
     } else {
       return result;
     }
   }
 
   static String executable = _executable();
   static String packageRoot = _packageRoot();
   static List<String> get executableArguments => _executableArguments();
 
   static Map<String, String> get environment {
-    var env = _environment();
-    if (env is OSError) {
-      throw env;
+    if (_environmentCache == null) {
+      var env = _environment();
+      if (env is !OSError) {
+        var isWindows = operatingSystem == 'windows';
+        var result = isWindows ? new _CaseInsensitiveStringMap() : new Map();
+        for (var str in env) {
+          // When running on Windows through cmd.exe there are strange
+          // environment variables that are used to record the current
+          // working directory for each drive and the exit code for the
+          // last command. As an example: '=A:=A:\subdir' records the
+          // current working directory on the 'A' drive.  In order to
+          // handle these correctly we search for a second occurrence of
+          // of '=' in the string if the first occurrence is at index 0.
+          var equalsIndex = str.indexOf('=');
+          if (equalsIndex == 0) {
+            equalsIndex = str.indexOf('=', 1);
+          }
+          assert(equalsIndex != -1);
+          result[str.substring(0, equalsIndex)] =
+              str.substring(equalsIndex + 1);
+        }
+        _environmentCache = new _UnmodifiableMap(result);
+      } else {
+        _environmentCache = env;
+      }
+    }
+
+    if (_environmentCache is OSError) {
+      throw _environmentCache;
     } else {
-      var isWindows = operatingSystem == 'windows';
-      var result = isWindows ? new _CaseInsensitiveStringMap() : new Map();
-      for (var str in env) {
-        // When running on Windows through cmd.exe there are strange
-        // environment variables that are used to record the current
-        // working directory for each drive and the exit code for the
-        // last command. As an example: '=A:=A:\subdir' records the
-        // current working directory on the 'A' drive.  In order to
-        // handle these correctly we search for a second occurrence of
-        // of '=' in the string if the first occurrence is at index 0.
-        var equalsIndex = str.indexOf('=');
-        if (equalsIndex == 0) {
-          equalsIndex = str.indexOf('=', 1);
-        }
-        assert(equalsIndex != -1);
-        result[str.substring(0, equalsIndex)] = str.substring(equalsIndex + 1);
-      }
-      return result;
+      return _environmentCache;
     }
   }
 
   static String get version => _version();
 }
 
 // Environment variables are case-insensitive on Windows. In order
 // to reflect that we use a case-insensitive string map on Windows.
 class _CaseInsensitiveStringMap<V> implements Map<String, V> {
   _CaseInsensitiveStringMap() : _map = new Map<String, V>();
@@ skipping 21 matching lines @@
   void clear() => _map.clear();
   void forEach(void f(String key, V value)) => _map.forEach(f);
   Iterable<String> get keys => _map.keys;
   Iterable<V> get values => _map.values;
   int get length => _map.length;
   bool get isEmpty => _map.isEmpty;
   bool get isNotEmpty => _map.isNotEmpty;
 
   Map<String, V> _map;
 }