All strong mode cleaning of dart2js.

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

Index: pkg/compiler/lib/src/js_backend/lookup_map_analysis.dart
diff --git a/pkg/compiler/lib/src/js_backend/lookup_map_analysis.dart b/pkg/compiler/lib/src/js_backend/lookup_map_analysis.dart
index 234042e18517049d03052afee941ae6942e74a26..4fddd0cdad2ce35be8358b5c8eb9de940f391b7d 100644
--- a/pkg/compiler/lib/src/js_backend/lookup_map_analysis.dart
+++ b/pkg/compiler/lib/src/js_backend/lookup_map_analysis.dart
@@ -307,10 +307,9 @@ class _LookupMapAnalysis implements LookupMapAnalysis {
       key.isPrimitive || _inUse.contains(key) || _overridesEquals(key);
 
   void _addClassUse(ClassEntity cls) {
-    ConstantValue key = _typeConstants.putIfAbsent(
-        cls,
-        () => _constantSystem.createType(
-            _commonElements, _elementEnvironment.getRawType(cls)));
+    TypeConstantValue f() => _constantSystem.createType(
+        _commonElements, _elementEnvironment.getRawType(cls));
+    ConstantValue key = _typeConstants.putIfAbsent(cls, f);
     _addUse(key);
   }