All strong mode cleaning of dart2js.

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

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /// Analysis to determine how to generate code for `LookupMap`s.
 library compiler.src.js_backend.lookup_map_analysis;
 
 import 'package:pub_semver/pub_semver.dart';
 
 import '../common.dart';
@@ skipping 289 matching lines @@
     return false;
   }
 
   /// Whether we need to preserve [key]. This is true for keys that are not
   /// candidates for tree-shaking in the first place (primitives and non-type
   /// const values overriding equals) and keys that we have seen in the program.
   bool _shouldKeep(ConstantValue key) =>
       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);
   }
 
   /// Record that [key] is used and update every lookup map that contains it.
   void _addUse(ConstantValue key) {
     if (_inUse.add(key)) {
       _pending[key]?.forEach((info) => info._markUsed(key));
       _pending.remove(key);
     }
   }
@@ skipping 192 matching lines @@
         original.fields[analysis._valueField] = keyValuePairs[1];
       }
     } else {
       original.fields[analysis._entriesField] =
           new ListConstantValue(listType, keyValuePairs);
     }
   }
 }
 
 final _validLookupMapVersionConstraint = new VersionConstraint.parse('^0.0.1');