fix temps that have the same name to have different Elements

lib/src/codegen/js_names.dart

Index: lib/src/codegen/js_names.dart
diff --git a/lib/src/codegen/js_names.dart b/lib/src/codegen/js_names.dart
index 9d03bc37cb69d7939447509fe15f37d92c391ac4..00ee437a6231bef9120e9a0ab4c880c9ac3e9e93 100644
--- a/lib/src/codegen/js_names.dart
+++ b/lib/src/codegen/js_names.dart
@@ -85,7 +85,10 @@ class _RenameVisitor extends VariableDeclarationVisitor {
 
   declare(Identifier node) {
     var id = identifierKey(node);
-    scope.declared.add(id);
+    var notAlreadyDeclared = scope.declared.add(id);
+    // Normal identifiers can be declared multiple times, because we don't
+    // implement block scope yet. However temps should only be declared once.
+    assert(notAlreadyDeclared || node is! TemporaryId);
     _markUsed(node, id, scope);
   }