Fix some type warnings.

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 136 matching lines @@
       Constant folded = operation.fold(receiver.constant);
       if (folded !== null) return graph.addConstant(folded);
     }
     return node;
   }
 
   HInstruction visitInvokeInterceptor(HInvokeInterceptor node) {
     if (node.isLengthGetter()) {
       HInstruction input = node.inputs[1];
       if (input.isConstantString()) {
-        StringConstant constant = input.constant;
+        HConstant constantInput = input;
+        StringConstant constant = constantInput.constant;
         return graph.addConstantInt(constant.length);
       } else if (input.isConstantList()) {
-        ListConstant constant = input.constant;
+        HConstant constantInput = input;
+        ListConstant constant = constantInput.constant;
         return graph.addConstantInt(constant.length);
       } else if (input.isConstantMap()) {
-        MapConstant constant = input.constant;
+        HConstant constantInput = input;
+        MapConstant constant = constantInput.constant;
         return graph.addConstantInt(constant.length);
       }
     }
     return node;
   }
 
   HInstruction visitInvokeDynamic(HInvokeDynamic node) {
     HType receiverType = node.receiver.propagatedType;
     if (receiverType.isNonPrimitive()) {
       HNonPrimitiveType type = receiverType;
@@ skipping 638 matching lines @@
         }
       }
       if (!canBeMoved) continue;
 
       // This is safe because we are running after GVN.
       // TODO(ngeoffray): ensure GVN has been run.
       set_.add(current);
     }
   }
 }