dart2js cps: Fix a couple of minor issues.

pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.dart

 // Copyright (c) 2014, 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.
 
 library tree_ir.optimization.statement_rewriter;
 
 import 'optimization.dart' show Pass;
 import '../tree_ir_nodes.dart';
 
 /**
@@ skipping 289 matching lines @@
            isEffectivelyConstant(node.value);
   }
 
   Statement visitExpressionStatement(ExpressionStatement stmt) {
     if (isEffectivelyConstantAssignment(stmt.expression) &&
         !usesRecentlyAssignedVariable(stmt.expression)) {
       Assign assign = stmt.expression;
       // Handle constant assignments specially.
       // They are always safe to propagate (though we should avoid duplication).
       // Moreover, they should not prevent other expressions from propagating.
-      if (assign.variable.readCount <= 1) {
+      if (assign.variable.readCount == 1) {

[asgerf, 2015/06/29 14:09:12]

This matters when there is a dead variable due to optimizations being disabled.

         // A single-use constant should always be propagted to its use site.
         constantEnvironment[assign.variable] = assign.value;
         --assign.variable.writeCount;
         return visitStatement(stmt.next);
       } else {
         // With more than one use, we cannot propagate the constant.
         // Visit the following statement without polluting [environment] so
         // that any preceding non-constant assignments might still propagate.
         stmt.next = visitStatement(stmt.next);
         assign.value = visitExpression(assign.value);
@@ skipping 718 matching lines @@
   IsVariableUsedVisitor(this.variable);
 
   visitVariableUse(VariableUse node) {
     if (node.variable == variable) {
       wasFound = true;
     }
   }
 
   visitInnerFunction(FunctionDefinition node) {}
 }