Reenable GVN for fields.

lib/compiler/implementation/ssa/nodes.dart

 // Copyright (c) 2012, 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 HVisitor<R> {
   R visitAdd(HAdd node);
   R visitBitAnd(HBitAnd node);
   R visitBitNot(HBitNot node);
   R visitBitOr(HBitOr node);
   R visitBitXor(HBitXor node);
@@ skipping 1254 matching lines @@
       if (name == const SourceString('add')
           || name == const SourceString('removeLast')) {
         return HType.MUTABLE_ARRAY;
       }
     }
     return HType.UNKNOWN;
   }
 
   void prepareGvn() {
     if (isLengthGetterOnStringOrArray()) {
+      setUseGvn();
       clearAllSideEffects();
+      setDependsOnSomething();
     } else {
       setAllSideEffects();
     }
   }
 
   int typeCode() => 4;
   bool typeEquals(other) => other is HInvokeInterceptor;
   bool dataEquals(HInvokeInterceptor other) {
     return getter == other.getter && name == other.name;
   }
@@ skipping 10 matching lines @@
 
   HFieldGet(Element element, HInstruction receiver,
             [this.isFinalOrConst = false])
       : super(element, <HInstruction>[receiver]);
 
   HInstruction get receiver() => inputs[0];
 
   accept(HVisitor visitor) => visitor.visitFieldGet(this);
 
   void prepareGvn() {
+    setUseGvn();
     clearAllSideEffects();

[floitsch, 2012/07/20 18:25:20]

The original version (when GVN was enabled) did not have the
"clearAllSideEffects()" call. Please confirm that it is correct to keep it.

+    if (!isFinalOrConst) setDependsOnSomething();
   }
 
   int typeCode() => 27;
   bool typeEquals(other) => other is HFieldGet;
   bool dataEquals(HFieldGet other) => element == other.element;
   String toString() => "FieldGet $element";
 }
 
 class HFieldSet extends HFieldAccess {
   HFieldSet(Element element, HInstruction receiver, HInstruction value)
@@ skipping 1341 matching lines @@
   HBasicBlock get start() => expression.start;
   HBasicBlock get end() {
     // We don't create a switch block if there are no cases.
     assert(!statements.isEmpty());
     return statements.last().end;
   }
 
   bool accept(HStatementInformationVisitor visitor) =>
       visitor.visitSwitchInfo(this);
 }