Make the Platform.environment return value an unmodifiable map

sdk/lib/io/platform_impl.dart

Index: sdk/lib/io/platform_impl.dart
diff --git a/sdk/lib/io/platform_impl.dart b/sdk/lib/io/platform_impl.dart
index 40fe2f0cf79a7b557d2dfa1c3385262426fd1c19..e85ce5c42c21f66f3169e13e01ff617fd2754dee 100644
--- a/sdk/lib/io/platform_impl.dart
+++ b/sdk/lib/io/platform_impl.dart
@@ -15,6 +15,10 @@ class _Platform {
   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();
@@ -45,28 +49,37 @@ class _Platform {
   static List<String> get executableArguments => _executableArguments();
 
   static Map<String, String> get environment {
-    var env = _environment();
-    if (env is OSError) {
-      throw env;
-    } 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);
+    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);
         }
-        assert(equalsIndex != -1);
-        result[str.substring(0, equalsIndex)] = str.substring(equalsIndex + 1);
+        _environmentCache = new _UnmodifiableMap(result);
+      } else {
+        _environmentCache = env;
       }
-      return result;
+    }
+
+    if (_environmentCache is OSError) {
+      throw _environmentCache;
+    } else {
+      return _environmentCache;
     }
   }