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 361 matching lines @@
       // 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.
+          constantEnvironment.remove(assign.variable);

[asgerf, 2015/07/02 14:55:01]

This matters for variables that have a single use but that use could not be
inlined into the successor statement because of a try block.

When the use is visited later, this code has already given up and left the
constant assignment here.

When we get to the use, it is important that it does not try to process the
constant again.

[floitsch, 2015/07/02 16:01:24]

Can you put this as a comment?

[asgerf, 2015/07/02 16:09:45]

Done.

           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);
   }
 }