dart2js: keep track of non-final static fields.

pkg/compiler/lib/src/universe/universe.dart

Index: pkg/compiler/lib/src/universe/universe.dart
diff --git a/pkg/compiler/lib/src/universe/universe.dart b/pkg/compiler/lib/src/universe/universe.dart
index 712b870408a4238867c9f677ddb8514f52f9359a..c1ad13fc1cd8fdc31f2862a321c4bf915ead235d 100644
--- a/pkg/compiler/lib/src/universe/universe.dart
+++ b/pkg/compiler/lib/src/universe/universe.dart
@@ -37,6 +37,11 @@ class Universe {
   /// Invariant: Elements are declaration elements.
   final Set<ClassElement> _allInstantiatedClasses = new Set<ClassElement>();
 
+  /// The set of all referenced static fields.
+  ///
+  /// Invariant: Elements are declaration elements.
+  final Set<FieldElement> allReferencedStaticFields = new Set<FieldElement>();
+
   /**
    * Documentation wanted -- johnniwinther
    *
@@ -185,6 +190,13 @@ class Universe {
     return type;
   }
 
+  void registerStaticFieldUse(FieldElement staticField) {
+    assert(Elements.isStaticOrTopLevel(staticField) && staticField.isField);
+    assert(staticField.isDeclaration);
+
+    allReferencedStaticFields.add(staticField);
+  }
+
   void forgetElement(Element element, Compiler compiler) {
     allClosures.remove(element);
     slowDirectlyNestedClosures(element).forEach(compiler.forgetElement);