Make field-get/set work without elements.

lib/compiler/implementation/ssa/optimize.dart

Index: lib/compiler/implementation/ssa/optimize.dart
diff --git a/lib/compiler/implementation/ssa/optimize.dart b/lib/compiler/implementation/ssa/optimize.dart
index 2b4831ad2a2097aa905c57f98ba09560826f80c2..ef7273adb755e001b8be7f22cdfdd358386d5dac 100644
--- a/lib/compiler/implementation/ssa/optimize.dart
+++ b/lib/compiler/implementation/ssa/optimize.dart
@@ -592,7 +592,8 @@ class SsaConstantFolder extends HBaseVisitor implements OptimizationPhase {
           break;
       }
     }
-    return new HFieldGet(field, node.inputs[0], isFinalOrConst: isFinalOrConst);
+    return new HFieldGet.withElement(
+        field, node.inputs[0], isFinalOrConst: isFinalOrConst);
   }
 
   HInstruction visitInvokeDynamicSetter(HInvokeDynamicSetter node) {
@@ -603,7 +604,7 @@ class SsaConstantFolder extends HBaseVisitor implements OptimizationPhase {
     if (type === null) return node;
     Element field = compiler.world.locateSingleField(type, node.name);
     if (field === null) return node;
-    return new HFieldSet(field, node.inputs[0], node.inputs[1]);
+    return new HFieldSet.withElement(field, node.inputs[0], node.inputs[1]);
   }
 
   HInstruction visitStringConcat(HStringConcat node) {