Assign invalid name to globalized closures.

sdk/lib/_internal/compiler/implementation/js_emitter/code_emitter_task.dart

Index: sdk/lib/_internal/compiler/implementation/js_emitter/code_emitter_task.dart
diff --git a/sdk/lib/_internal/compiler/implementation/js_emitter/code_emitter_task.dart b/sdk/lib/_internal/compiler/implementation/js_emitter/code_emitter_task.dart
index 9507f3e420a1eab579778c8bcddcb86d481f1680..7e2fdb1e74b592318277fa246dc0f083ecfb0f9e 100644
--- a/sdk/lib/_internal/compiler/implementation/js_emitter/code_emitter_task.dart
+++ b/sdk/lib/_internal/compiler/implementation/js_emitter/code_emitter_task.dart
@@ -679,6 +679,12 @@ class CodeEmitterTask extends CompilerTask {
       // marking the function as invokable by reflection.
       return '$name=';
     }
+    if (elementOrSelector is Element && elementOrSelector.isClosure()) {
+      // Closures are synthesized and their name might conflict with existing
+      // globals. Assign an illegal name, and make sure they don't clash
+      // with each other.
+      return " $mangledName";
+    }
     if (elementOrSelector is Selector
         || elementOrSelector.isFunction()
         || elementOrSelector.isConstructor()) {