Update pkg/serialization to use Symbols and synchronous mirrors

pkg/serialization/test/serialization_test.dart

Index: pkg/serialization/test/serialization_test.dart
diff --git a/pkg/serialization/test/serialization_test.dart b/pkg/serialization/test/serialization_test.dart
index 2987fe09033eda36c7af28e189ec2bc621c7b8ea..0729fdd10ec87ad0b6087c3f8bb74f11fdafaf9c 100644
--- a/pkg/serialization/test/serialization_test.dart
+++ b/pkg/serialization/test/serialization_test.dart
@@ -123,9 +123,16 @@ main() {
     var stream = new Stream([3,4,5]);
     expect((stream..next()).next(), 4);
     expect(stream.position, 2);
+    // The Symbol class does not allow us to create symbols for private
+    // variables. However, the mirror system uses them. So we get the symbol
+    // we want from the mirror.
+    // TODO(alanknight): Either delete this test and decide we shouldn't
+    // attempt to access private variables or fix this properly.
+    var _collectionSym = reflect(stream).type.variables.keys.firstWhere(
+        (x) => MirrorSystem.getName(x) == "_collection");
     var s = new Serialization()
       ..addRuleFor(stream,
-          constructorFields: ['_collection']);
+          constructorFields: [_collectionSym]);
     var state = states(stream, s).first;
     // Define names for the variable offsets to make this more readable.
     var _collection = 0, position = 1;
@@ -211,7 +218,7 @@ main() {
     var w = new Writer(s);
     w.trace = new Trace(w);
     w.write(n1);
-    expect(w.states.length, 5); // prims, lists, essential lists, basic
+    expect(w.states.length, 6); // prims, lists, essential lists, basic, symbol
     var children = 0, name = 1, parent = 2;
     var nodeRule = s.rules.firstWhere((x) => x is BasicRule);
     List rootNode = w.states[nodeRule.number].where(