Fix dart2js by copying keys of a map that is modified under us.

sdk/lib/_internal/compiler/implementation/closure.dart

Index: sdk/lib/_internal/compiler/implementation/closure.dart
diff --git a/sdk/lib/_internal/compiler/implementation/closure.dart b/sdk/lib/_internal/compiler/implementation/closure.dart
index 201817f3dfa03c966db0b5326e897a60dcc71b30..2733cfc03fbfdf320e80c6014546a5eea08203e7 100644
--- a/sdk/lib/_internal/compiler/implementation/closure.dart
+++ b/sdk/lib/_internal/compiler/implementation/closure.dart
@@ -259,7 +259,7 @@ class ClosureTranslator extends Visitor {
       Map<Element, Element> freeVariableMapping = data.freeVariableMapping;
       // We get a copy of the keys and iterate over it, to avoid modifications
       // to the map while iterating over it.
-      freeVariableMapping.keys.forEach((Element fromElement) {
+      freeVariableMapping.keys.toList().forEach((Element fromElement) {
         assert(fromElement == freeVariableMapping[fromElement]);
         Element updatedElement = capturedVariableMapping[fromElement];
         assert(updatedElement != null);