Support overriding fields with fields.

dart/lib/compiler/implementation/ssa/optimize.dart

 // Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 interface OptimizationPhase {
   String get name();
   void visitGraph(HGraph graph);
 }
 
 class SsaOptimizerTask extends CompilerTask {
@@ skipping 553 matching lines @@
     }
     HType combinedType = value.propagatedType.intersection(node.propagatedType);
     return (combinedType == value.propagatedType) ? value : node;
   }
 
   HInstruction visitInvokeDynamicGetter(HInvokeDynamicGetter node) {
     HInstruction receiver = node.inputs[0];
     if (!receiver.propagatedType.isUseful()) return node;
     Type type = receiver.propagatedType.computeType(compiler);
     if (type === null) return node;
-    if (!compiler.world.isOnlyFields(type, node.name)) return node;
-    ClassElement cls = type.element;
-    if (cls === null) return node;
-    Element element = cls.lookupLocalMember(node.name);
-    // If a subclass overrides a final field then there will be two fields,
-    // and here the final field will be used.
-    if (element === null) return node;
-    Modifiers modifiers = element.modifiers;
+    Element field = compiler.world.locateSingleField(type, node.name);
+    if (field === null) return node;
+    Modifiers modifiers = field.modifiers;
     bool isFinalOrConst = false;
     if (modifiers != null) {
       isFinalOrConst = modifiers.isFinal() || modifiers.isConst();
     }
-    return new HFieldGet(node.name,
-                         node.inputs[0],
-                         isFinalOrConst: isFinalOrConst);
+    return new HFieldGet(field, node.inputs[0], isFinalOrConst: isFinalOrConst);
   }
 
   HInstruction visitInvokeDynamicSetter(HInvokeDynamicSetter node) {
     HInstruction receiver = node.inputs[0];
     if (!receiver.propagatedType.isUseful()) return node;
     Type type = receiver.propagatedType.computeType(compiler);
     if (type === null) return node;
-    if (!compiler.world.isOnlyFields(type, node.name)) return node;
-    return new HFieldSet(node.name, node.inputs[0], node.inputs[1]);
+    Element field = compiler.world.locateSingleField(type, node.name);
+    if (field === null) return node;
+    return new HFieldSet(field, node.inputs[0], node.inputs[1]);
   }
 }
 
 class SsaCheckInserter extends HBaseVisitor implements OptimizationPhase {
   final String name = "SsaCheckInserter";
   Element lengthInterceptor;
 
   SsaCheckInserter(JavaScriptBackend backend) {
     SourceString lengthString = const SourceString('length');
     lengthInterceptor =
@@ skipping 522 matching lines @@
         // the if block terminates. So any use of the instruction
         // after the join block should be changed to the new
         // instruction.
         changeUsesDominatedBy(ifUser.joinBlock, input, convertedType);
       }
       // TODO(ngeoffray): Also change uses for the then block on a HType
       // that knows it is not of a specific Type.
     }
   }
 }