dart2js cps: Fix bug in assignment propagation.

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

Index: pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.dart
diff --git a/pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.dart b/pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.dart
index 8025ec3b2ff1feaf2aa896ccd5a63cab71b64b0d..be59dc90692297fe4217949f62c5415defa83806 100644
--- a/pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.dart
+++ b/pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.dart
@@ -377,7 +377,11 @@ class StatementRewriter extends Transformer implements Pass {
         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;