Ignore non-UTF8 strings in the environment

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 75278443388f638eed4f792de401ef8c68604cbf..92d56994a3563e3e4fd7d1d27b189e0c51014edc 100644
--- a/sdk/lib/io/platform_impl.dart
+++ b/sdk/lib/io/platform_impl.dart
@@ -66,6 +66,9 @@ class _Platform {
             ? new _CaseInsensitiveStringMap<String>()
             : new Map<String, String>();
         for (var str in env) {
+          if (str == null) {
+            continue;
+          }
           // The Strings returned by [_environment()] are expected to be
           // valid environment entries, but exceptions have been seen
           // (e.g., an entry of just '=' has been seen on OS/X).