Make field-get/set work without elements.

lib/compiler/implementation/ssa/tracer.dart

Index: lib/compiler/implementation/ssa/tracer.dart
diff --git a/lib/compiler/implementation/ssa/tracer.dart b/lib/compiler/implementation/ssa/tracer.dart
index eab32e1e22c0350c9a543adf4e471ab5e3a4fbd9..154e2fb860d9f86a0d88437231fade55db6eda5a 100644
--- a/lib/compiler/implementation/ssa/tracer.dart
+++ b/lib/compiler/implementation/ssa/tracer.dart
@@ -259,13 +259,12 @@ class HInstructionStringifier implements HVisitor<String> {
   String visitExit(HExit node) => "exit";
 
   String visitFieldGet(HFieldGet node) {
-    String fieldName = node.element.name.slowToString();
-    return 'get ${temporaryId(node.receiver)}.$fieldName';
+    return 'get ${temporaryId(node.receiver)}.${node.fieldName.slowToString()}';
   }
 
   String visitFieldSet(HFieldSet node) {
     String valueId = temporaryId(node.value);
-    String fieldName = node.element.name.slowToString();
+    String fieldName = node.fieldName.slowToString();
     return 'set ${temporaryId(node.receiver)}.$fieldName to $valueId';
   }