dart2js cps: Fix bug in assignment propagation.

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 359 matching lines @@
       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) {
         // A single-use constant should always be propagated to its use site.
         constantEnvironment[assign.variable] = assign.value;
         Statement next = visitStatement(stmt.next);
         popDominatingAssignment(leftHand);
         if (assign.variable.readCount > 0) {
-          // The assignment could not be propagated.
+          // The assignment could not be propagated into the successor, either
+          // because it has an unsafe variable use (see [hasUnsafeVariableUse])
+          // or because the use is outside the current try block, and we do
+          // not currently support constant propagation out of a try block.
+          constantEnvironment.remove(assign.variable);
           assign.value = visitExpression(assign.value);
           stmt.next = next;
           return stmt;
         } else {
           --assign.variable.writeCount;
           return next;
         }
       } else {
         // With more than one use, we cannot propagate the constant.
         // Visit the following statement without polluting [environment] so
@@ skipping 748 matching lines @@
   VariableUseVisitor(this.callback);
 
   visitVariableUse(VariableUse use) => callback(use);
 
   visitInnerFunction(FunctionDefinition node) {}
 
   static void visit(Expression node, VariableUseCallback callback) {
     new VariableUseVisitor(callback).visitExpression(node);
   }
 }