diff --git a/pkg/dart2wasm/lib/class_info.dart b/pkg/dart2wasm/lib/class_info.dart
index 12a9a40..04015cf 100644
--- a/pkg/dart2wasm/lib/class_info.dart
+++ b/pkg/dart2wasm/lib/class_info.dart
@@ -149,8 +149,9 @@
 
   /// The class whose struct is used as the type for variables of this type.
   /// This is a type which is a superclass of all subtypes of this type.
-  late final ClassInfo repr = upperBound(
-      implementedBy.map((c) => identical(c, this) ? this : c.repr).toSet());
+  ClassInfo get repr => _repr!;
+
+  ClassInfo? _repr;
 
   /// All classes which implement this class. This is used to compute `repr`.
   final List<ClassInfo> implementedBy = [];
@@ -185,25 +186,22 @@
       ];
 }
 
-ClassInfo upperBound(Set<ClassInfo> classes) {
-  while (classes.length > 1) {
-    Set<ClassInfo> newClasses = {};
-    int minDepth = 999999999;
-    int maxDepth = 0;
-    for (ClassInfo info in classes) {
-      minDepth = min(minDepth, info.depth);
-      maxDepth = max(maxDepth, info.depth);
+ClassInfo upperBound(ClassInfo a, ClassInfo b) {
+  if (a.depth < b.depth) {
+    while (b.depth > a.depth) {
+      b = b.superInfo!;
     }
-    int targetDepth = minDepth == maxDepth ? minDepth - 1 : minDepth;
-    for (ClassInfo info in classes) {
-      while (info.depth > targetDepth) {
-        info = info.superInfo!;
-      }
-      newClasses.add(info);
+  } else {
+    while (a.depth > b.depth) {
+      a = a.superInfo!;
     }
-    classes = newClasses;
   }
-  return classes.single;
+  assert(a.depth == b.depth);
+  while (a != b) {
+    a = a.superInfo!;
+    b = b.superInfo!;
+  }
+  return a;
 }
 
 /// Constructs the Wasm type hierarchy.
@@ -439,7 +437,7 @@
     // `0` is occupied by artificial non-Dart top class.
     const int firstClassId = 1;
 
-    translator.classIdNumbering =
+    final classIdNumbering = translator.classIdNumbering =
         ClassIdNumbering._number(translator, masqueraded, firstClassId);
     final classIds = translator.classIdNumbering.classIds;
     final dfsOrder = translator.classIdNumbering.dfsOrder;
@@ -476,6 +474,25 @@
       }
     }
 
+    // Create representations of the classes (i.e. wasm representation used to
+    // represent objects of that dart type).
+    for (final cls in dfsOrder) {
+      ClassInfo? representation;
+      for (final range in classIdNumbering.getConcreteClassIdRanges(cls)) {
+        for (int classId = range.start; classId <= range.end; ++classId) {
+          final current = translator.classes[classId];
+          if (representation == null) {
+            representation = current;
+            continue;
+          }
+          representation = upperBound(representation, current);
+        }
+      }
+      final classId = classIdNumbering.classIds[cls]!;
+      final info = translator.classes[classId];
+      info._repr = representation ?? translator.classes[classId];
+    }
+
     // Now that the representation types for all classes have been computed,
     // fill in the types of the fields in the generated Wasm structs.
     for (final info in translator.classesSupersFirst) {
diff --git a/pkg/dart2wasm/lib/closures.dart b/pkg/dart2wasm/lib/closures.dart
index 841a49f..ce137fd 100644
--- a/pkg/dart2wasm/lib/closures.dart
+++ b/pkg/dart2wasm/lib/closures.dart
@@ -1041,6 +1041,7 @@
   final Map<TreeNode, Capture> captures = {};
   bool isThisCaptured = false;
   final Map<FunctionNode, Lambda> lambdas = {};
+  late final w.RefType? nullableThisType;
 
   // This [TreeNode] is the context owner, and can be a [FunctionNode],
   // [Constructor], [ForStatement], [DoStatement] or a [WhileStatement].
@@ -1048,7 +1049,12 @@
   final Set<FunctionDeclaration> closurizedFunctions = {};
 
   Closures(this.translator, Member member)
-      : enclosingClass = member.enclosingClass;
+      : enclosingClass = member.enclosingClass {
+    final hasThis = member is Constructor || member.isInstanceMember;
+    nullableThisType = hasThis
+        ? translator.preciseThisFor(member, nullable: true) as w.RefType
+        : null;
+  }
 
   w.ModuleBuilder get m => translator.m;
 
@@ -1104,8 +1110,7 @@
         }
         if (context.containsThis) {
           assert(enclosingClass != null);
-          struct.fields.add(
-              w.FieldType(translator.classInfo[enclosingClass!]!.nullableType));
+          struct.fields.add(w.FieldType(nullableThisType!));
         }
         for (VariableDeclaration variable in context.variables) {
           int index = struct.fields.length;
diff --git a/pkg/dart2wasm/lib/code_generator.dart b/pkg/dart2wasm/lib/code_generator.dart
index 51161ec..91084f5 100644
--- a/pkg/dart2wasm/lib/code_generator.dart
+++ b/pkg/dart2wasm/lib/code_generator.dart
@@ -724,23 +724,15 @@
   /// parameter if there are no type parameters).
   int _initializeThis(Reference reference) {
     Member member = reference.asMember;
-    bool hasThis =
+    final hasThis =
         member.isInstanceMember || reference.isConstructorBodyReference;
     if (hasThis) {
       thisLocal = paramLocals[0];
-      assert(!thisLocal!.type.nullable);
-      Class cls = member.enclosingClass!;
-      w.StorageType? builtin = translator.builtinTypes[cls];
-      w.ValueType thisType = translator.boxedClasses.containsKey(builtin)
-          ? builtin as w.ValueType
-          : translator.classInfo[cls]!.nonNullableType;
-      if (translator.needsConversion(thisLocal!.type, thisType) &&
-          !(cls == translator.objectInfo.cls ||
-              cls == translator.ffiPointerClass ||
-              translator.isWasmType(cls))) {
-        preciseThisLocal = addLocal(thisType);
+      final preciseThisType = translator.preciseThisFor(member);
+      if (translator.needsConversion(thisLocal!.type, preciseThisType)) {
+        preciseThisLocal = addLocal(preciseThisType);
         b.local_get(thisLocal!);
-        translator.convertType(function, thisLocal!.type, thisType);
+        translator.convertType(function, thisLocal!.type, preciseThisType);
         b.local_set(preciseThisLocal!);
       } else {
         preciseThisLocal = thisLocal!;
@@ -1739,14 +1731,19 @@
     w.ValueType preciseThisType = preciseThisLocal!.type;
     assert(!thisType.nullable);
     assert(!preciseThisType.nullable);
-    if (!thisType.isSubtypeOf(expectedType) &&
-        preciseThisType.isSubtypeOf(expectedType)) {
-      b.local_get(preciseThisLocal!);
-      return preciseThisType;
-    } else {
+    if (thisType.isSubtypeOf(expectedType)) {
       b.local_get(thisLocal!);
       return thisType;
     }
+    if (preciseThisType.isSubtypeOf(expectedType)) {
+      b.local_get(preciseThisLocal!);
+      return preciseThisType;
+    }
+    // A user of `this` may have more precise type information, in which case
+    // we downcast it here.
+    b.local_get(thisLocal!);
+    translator.convertType(function, thisType, expectedType);
+    return expectedType;
   }
 
   @override
@@ -1787,7 +1784,7 @@
         _lookupSuperTarget(node.interfaceTarget, setter: false).reference;
     w.FunctionType targetFunctionType =
         translator.functions.getFunctionType(target);
-    w.ValueType receiverType = targetFunctionType.inputs[0];
+    final w.ValueType receiverType = translator.preciseThisFor(target.asMember);
 
     // When calling `==` and the argument is potentially nullable, check if the
     // argument is `null`.
diff --git a/pkg/dart2wasm/lib/dynamic_forwarders.dart b/pkg/dart2wasm/lib/dynamic_forwarders.dart
index 2b169db..ae624c9 100644
--- a/pkg/dart2wasm/lib/dynamic_forwarders.dart
+++ b/pkg/dart2wasm/lib/dynamic_forwarders.dart
@@ -104,8 +104,6 @@
         if (targetMember.isAbstract) {
           continue;
         }
-        final targetClass = targetMember.enclosingClass!;
-        final targetClassInfo = translator.classInfo[targetClass]!;
 
         b.local_get(receiverLocal);
         b.struct_get(translator.topInfo.struct, FieldIndex.classId);
@@ -113,7 +111,6 @@
         b.i32_eq();
         b.if_();
 
-        final w.ValueType receiverType = targetClassInfo.nonNullableType;
         final Reference targetReference;
         if (targetMember is Procedure) {
           targetReference = targetMember.isGetter
@@ -128,7 +125,8 @@
         final w.BaseFunction targetFunction =
             translator.functions.getFunction(targetReference);
         b.local_get(receiverLocal);
-        translator.convertType(function, receiverLocal.type, receiverType);
+        translator.convertType(
+            function, receiverLocal.type, targetFunction.type.inputs.first);
         b.call(targetFunction);
         // Box return value if needed
         translator.convertType(function, targetFunction.type.outputs.single,
@@ -528,8 +526,6 @@
         if (targetMember is Procedure && !targetMember.isGetter) {
           continue;
         }
-        final targetClass = targetMember.enclosingClass!;
-        final targetClassInfo = translator.classInfo[targetClass]!;
 
         b.local_get(receiverLocal);
         b.struct_get(translator.topInfo.struct, FieldIndex.classId);
@@ -537,7 +533,6 @@
         b.i32_eq();
         b.if_();
 
-        final w.ValueType receiverType = targetClassInfo.nonNullableType;
         final Reference targetReference;
         if (targetMember is Procedure) {
           assert(targetMember.isGetter); // methods are skipped above
@@ -553,7 +548,8 @@
 
         // Get field value
         b.local_get(receiverLocal);
-        translator.convertType(function, receiverLocal.type, receiverType);
+        translator.convertType(
+            function, receiverLocal.type, targetFunction.type.inputs.first);
         b.call(targetFunction);
         translator.convertType(function, targetFunction.type.outputs.single,
             translator.topInfo.nullableType);
diff --git a/pkg/dart2wasm/lib/translator.dart b/pkg/dart2wasm/lib/translator.dart
index 04d33b7..c3ccbc9 100644
--- a/pkg/dart2wasm/lib/translator.dart
+++ b/pkg/dart2wasm/lib/translator.dart
@@ -902,6 +902,24 @@
     }
   }
 
+  w.ValueType preciseThisFor(Member member, {bool nullable = false}) {
+    assert(member.isInstanceMember || member is Constructor);
+
+    Class cls = member.enclosingClass!;
+    final w.StorageType? builtin = builtinTypes[cls];
+    final boxClass = boxedClasses[builtin];
+    if (boxClass != null) {
+      // We represent `this` as an unboxed type.
+      if (!nullable) return builtin as w.ValueType;
+      // Otherwise we use [boxClass] to represent `this`.
+      cls = boxClass;
+    }
+    final representationClassInfo = classInfo[cls]!.repr;
+    return nullable
+        ? representationClassInfo.nullableType
+        : representationClassInfo.nonNullableType;
+  }
+
   /// Get the Wasm table declared by [field], or `null` if [field] is not a
   /// declaration of a Wasm table.
   ///
diff --git a/tests/web/wasm_js_shared/omit_implicit_checks_test.dart b/tests/web/wasm_js_shared/omit_implicit_checks_test.dart
index 90d32a4..77868a7 100644
--- a/tests/web/wasm_js_shared/omit_implicit_checks_test.dart
+++ b/tests/web/wasm_js_shared/omit_implicit_checks_test.dart
@@ -36,7 +36,7 @@
 }
 
 void testDynamicCall() {
-  final dynamic a = kTrue ? (String a) => 'closure($a)' : A();
+  final dynamic a = kTrue ? (List a) => 'closure($a)' : A();
   Expect.equals('closure(B)', a(B()));
 }