Add fields to platform library, implement them on runtime dart.

runtime/bin/builtin.dart

Index: runtime/bin/builtin.dart
diff --git a/runtime/bin/builtin.dart b/runtime/bin/builtin.dart
index 91702ab412621dfa77515676f8fc0b17cfd065dd..893f90a7fb8a2d5878d7f53546864591dcc7b16a 100644
--- a/runtime/bin/builtin.dart
+++ b/runtime/bin/builtin.dart
@@ -310,3 +310,59 @@ String _filePathFromHttpUri(Uri uri) {
   _logResolution('# Path: $uri -> $uri');
   return uri.toString();
 }
+
+
+// platform library '_platform' implementation.
+_platform(int propertyIndex) {
+  const int kNumberOfProcessors = 1;
+  const int kPathSeparator = 2;
+  const int kOperatingSystem = 3;
+  const int kLocalHostname = 4;
+  const int kVersion = 5;
+  const int kEnvironment = 6;
+  const int kScript = 7;
+  const int kExecutable = 8;
+  const int kExecutableArguments = 9;
+  const int kPackageRoot = 10;
+  switch (propertyIndex) {
+    case kNumberOfProcessors:
+      return Platform.numberOfProcessors;
+      break;
+    case kPathSeparator:
+      return Platform.pathSeparator;
+      break;
+    case kOperatingSystem:
+      return Platform.operatingSystem;
+      break;
+    case kLocalHostname:
+      return Platform.localHostname;
+      break;
+    case kVersion:
+      return Platform.version;
+      break;
+    case kEnvironment:
+      return Platform.environment;
+      break;
+    case kScript:
+      return Platform.script;
+      break;
+    case kExecutable:
+      return Platform.executable;
+      break;
+    case kExecutableArguments:
+      return Platform.executableArguments;
+      break;
+    case kPackageRoot:
+      return Platform.packageRoot;
+      break;
+    default:
+      throw new UnimplementedError(
+          "Unknown property $propertyIndex of platform");
+      break;
+  }
+}
+
+_getPlatform() => _platform;
+
+
+