Make the Platform.environment return value an unmodifiable map

runtime/bin/process_patch.dart

Index: runtime/bin/process_patch.dart
diff --git a/runtime/bin/process_patch.dart b/runtime/bin/process_patch.dart
index b83ec1b60114c10cee1aa3ef05972e036e39937d..cab5b49f3c666af63e4ca081b24f1ca7b3cdfa66 100644
--- a/runtime/bin/process_patch.dart
+++ b/runtime/bin/process_patch.dart
@@ -198,7 +198,7 @@ class _ProcessImpl extends NativeFieldWrapperClass1 implements Process {
       throw new ArgumentError("Environment is not a map: $environment");
     }
     if (identical(true, includeParentEnvironment)) {
-      environment = Platform.environment..addAll(environment);
+      environment = new Map.from(Platform.environment)..addAll(environment);
     }
     environment.forEach((key, value) {
       if (key is !String || value is !String) {