dart2js cps: Fix bug in pulling of assignments into a var.

pkg/compiler/lib/src/js_backend/codegen/codegen.dart

Index: pkg/compiler/lib/src/js_backend/codegen/codegen.dart
diff --git a/pkg/compiler/lib/src/js_backend/codegen/codegen.dart b/pkg/compiler/lib/src/js_backend/codegen/codegen.dart
index 185b56df8be60a0b5e6a2adf1b48269a266018f9..07c3ff1043cd2658e7e46eb0f808ef570c6ddbcc 100644
--- a/pkg/compiler/lib/src/js_backend/codegen/codegen.dart
+++ b/pkg/compiler/lib/src/js_backend/codegen/codegen.dart
@@ -121,6 +121,9 @@ class CodeGenerator extends tree_ir.StatementVisitor
       if (assign.op != null) break; // Compound assignment.
       js.VariableUse use = assign.leftHandSide;
 
+      // Do not touch non-local variables.
+      if (!usedVariableNames.contains(use.name)) break;
+
       // We cannot declare a variable more than once.
       if (!declaredVariables.add(use.name)) break;
 
@@ -144,6 +147,9 @@ class CodeGenerator extends tree_ir.StatementVisitor
       if (assign.op != null) break pullFromForLoop; // Compound assignment.
       js.VariableUse use = assign.leftHandSide;
 
+      // Do not touch non-local variables.
+      if (!usedVariableNames.contains(use.name)) break pullFromForLoop;
+
       // We cannot declare a variable more than once.
       if (!declaredVariables.add(use.name)) break pullFromForLoop;