Change Platform members to getters instead of methods.

runtime/bin/process_impl.dart

Index: runtime/bin/process_impl.dart
diff --git a/runtime/bin/process_impl.dart b/runtime/bin/process_impl.dart
index 1963451252dab384f41d1888d047993703f59785..b0092dc7318a87135b67498bf302c5f16d49cbb3 100644
--- a/runtime/bin/process_impl.dart
+++ b/runtime/bin/process_impl.dart
@@ -55,7 +55,7 @@ class _InteractiveProcess implements Process {
         throw new IllegalArgumentException("Non-string argument: $arg");
       }
       _arguments[i] = arguments[i];
-      if (Platform.operatingSystem() == 'windows') {
+      if (Platform.operatingSystem == 'windows') {
         _arguments[i] = _windowsArgumentEscape(_arguments[i]);
       }
     }