Revert "Improvements to the kernel tree shaker."

pkg/kernel/lib/transformations/treeshaker.dart

 // Copyright (c) 2016, 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.
 
 library kernel.tree_shaker;
 
 import '../ast.dart';
 import '../class_hierarchy.dart';
 import '../core_types.dart';
-import '../type_environment.dart';
 
 Program transformProgram(Program program) {
   new TreeShaker(program).transform(program);
   return program;
 }
 
 /// Tree shaking based on class hierarchy analysis.
 ///
 /// Any dynamic dispatch not on `this` is conservatively assumed to target
 /// any instantiated class that implements a member matching the selector.
 ///
 /// Member bodies are analyzed relative to a given "host class" which is the
 /// concrete type of `this` (or null if in static context), so dispatches on
 /// `this` can be resolved more precisely.
 ///
 /// The tree shaker computes the following in a fixed-point iteration:
 /// - a set of instantiated classes
 /// - for each member, a set of potential host classes
 /// - a set of names used in dynamic dispatch not on `this`
 ///
-/// If the `dart:mirrors` library is used then nothing will be tree-shaken.
+/// The `dart:mirrors` library is not supported.
 //
 // TODO(asgerf): Shake off parts of the core libraries based on the Target.
 // TODO(asgerf): Tree shake unused instance fields.
 class TreeShaker {
   final Program program;
   final ClassHierarchy hierarchy;
   final CoreTypes coreTypes;
-  final bool strongMode;
 
-  /// Map from classes to set of names that have been dispatched with that class
-  /// as the static receiver type (meaning any subtype of that class can be
-  /// the potential concrete receiver).
-  ///
-  /// The map is implemented as a list, indexed by
-  /// [ClassHierarchy.getClassIndex].
-  final List<Set<Name>> _dispatchedNames;
-
-  /// Map from names to the set of classes that might be the concrete receiver
-  /// of a call with the given name.
-  final Map<Name, ClassSet> _receiversOfName = <Name, ClassSet>{};
+  /// Names used in a dynamic dispatch invocation that could not be resolved
+  /// to a concrete target (i.e. not on `this`).
+  final Set<Name> _dispatchedNames = new Set<Name>();
 
   /// Instance members that are potential targets for dynamic dispatch, but
   /// whose name has not yet been seen in a dynamic dispatch invocation.
   ///
   /// The map is indexed by the name of the member, and value is a list of
   /// interleaved (host class, member) pairs.
   final Map<Name, List<TreeNode>> _dispatchTargetCandidates =
       <Name, List<TreeNode>>{};
 
   /// Map from classes to the set of members that are reachable with that
@@ skipping 18 matching lines @@
   /// See [ClassRetention].
   final List<ClassRetention> _classRetention;
 
   /// Interleaved (host class, member) pairs that are reachable but have not yet
   /// been analyzed for more uses.
   final List<TreeNode> _worklist = new List<TreeNode>();
 
   /// Classes whose interface can be used by external code to invoke user code.
   final Set<Class> _escapedClasses = new Set<Class>();
 
-  /// Members that have been overridden by a member whose concrete body is
-  /// needed.  These must be preserved in order to maintain interface targets
-  /// for typed calls.
-  final Set<Member> _overriddenMembers = new Set<Member>();
-
-  final List<Expression> _typedCalls = <Expression>[];
-
   /// AST visitor for finding static uses and dynamic dispatches in code.
   _TreeShakerVisitor _visitor;
 
   /// AST visitor for analyzing type annotations on external members.
   _ExternalTypeVisitor _covariantVisitor;
   _ExternalTypeVisitor _contravariantVisitor;
-  _ExternalTypeVisitor _invariantVisitor;
+  _ExternalTypeVisitor _bivariantVisitor;
 
-  Library _mirrorsLibrary;
+  TreeShaker(Program program, {ClassHierarchy hierarchy, CoreTypes coreTypes})
+      : this._internal(program, hierarchy ?? new ClassHierarchy(program),
+            coreTypes ?? new CoreTypes(program));
 
-  /// Set to true if any use of the `dart:mirrors` API is found.
-  bool isUsingMirrors = false;
-
-  TreeShaker(Program program,
-      {ClassHierarchy hierarchy, CoreTypes coreTypes, bool strongMode: false})
-      : this._internal(program, hierarchy ?? new ClassHierarchy(program),
-            coreTypes ?? new CoreTypes(program), strongMode);
-
-  bool isMemberBodyUsed(Member member) {
+  bool isMemberUsed(Member member) {
     return _usedMembers.containsKey(member);
   }
 
-  bool isMemberOverridden(Member member) {
-    return _overriddenMembers.contains(member);
-  }
-
-  bool isMemberUsed(Member member) {
-    return isMemberBodyUsed(member) || isMemberOverridden(member);
-  }
-
   bool isInstantiated(Class classNode) {
     return getClassRetention(classNode).index >= ClassRetention.Instance.index;
   }
 
   bool isHierarchyUsed(Class classNode) {
     return getClassRetention(classNode).index >= ClassRetention.Hierarchy.index;
   }
 
   ClassRetention getClassRetention(Class classNode) {
     int index = hierarchy.getClassIndex(classNode);
     return _classRetention[index];
   }
 
   /// Applies the tree shaking results to the program.
   ///
   /// This removes unused classes, members, and hierarchy data.
   void transform(Program program) {
-    if (isUsingMirrors) return; // Give up if using mirrors.
     new _TreeShakingTransformer(this).transform(program);
   }
 
-  TreeShaker._internal(
-      this.program, ClassHierarchy hierarchy, this.coreTypes, this.strongMode)
+  TreeShaker._internal(this.program, ClassHierarchy hierarchy, this.coreTypes)
       : this.hierarchy = hierarchy,
-        this._dispatchedNames = new List<Set<Name>>(hierarchy.classes.length),
         this._usedMembersWithHost =
             new List<Set<Member>>(hierarchy.classes.length),
         this._classRetention = new List<ClassRetention>.filled(
             hierarchy.classes.length, ClassRetention.None) {
     _visitor = new _TreeShakerVisitor(this);
     _covariantVisitor = new _ExternalTypeVisitor(this, isCovariant: true);
     _contravariantVisitor =
         new _ExternalTypeVisitor(this, isContravariant: true);
-    _invariantVisitor = new _ExternalTypeVisitor(this,
+    _bivariantVisitor = new _ExternalTypeVisitor(this,
         isCovariant: true, isContravariant: true);
-    _mirrorsLibrary = coreTypes.getCoreLibrary('dart:mirrors');
-    try {
-      _build();
-    } on _UsingMirrorsException {
-      isUsingMirrors = true;
-    }
+    _build();
   }
 
   void _build() {
     if (program.mainMethod == null) {
       throw 'Cannot perform tree shaking on a program without a main method';
     }
     if (program.mainMethod.function.positionalParameters.length > 0) {
       // The main method takes a List<String> as argument.
       _addInstantiatedExternalSubclass(coreTypes.listClass);
       _addInstantiatedExternalSubclass(coreTypes.stringClass);
     }
-    _addDispatchedName(hierarchy.rootClass, new Name('noSuchMethod'));
+    _addDispatchedName(new Name('noSuchMethod'));
     _addPervasiveUses();
     _addUsedMember(null, program.mainMethod);
     _iterateWorklist();
-
-    // Mark overridden members in order to preserve abstract members as
-    // necessary.
-    if (strongMode) {
-      for (int i = hierarchy.classes.length - 1; i >= 0; --i) {
-        Class class_ = hierarchy.classes[i];
-        if (isHierarchyUsed(class_)) {
-          hierarchy.forEachOverridePair(class_,
-              (Member ownMember, Member superMember, bool isSetter) {
-            if (isMemberBodyUsed(ownMember) ||
-                _overriddenMembers.contains(ownMember)) {
-              _overriddenMembers.add(superMember);
-              // Ensure the types mentioned in the member can be preserved.
-              _visitor.visitMemberInterface(superMember);
-            }
-          });
-        }
-      }
-      // Marking members as overridden should not cause new code to become
-      // reachable.
-      assert(_worklist.isEmpty);
-    }
   }
 
   /// Registers some extremely commonly used core classes as instantiated, so
   /// we don't have to register them for every use we find.
   void _addPervasiveUses() {
     _addInstantiatedExternalSubclass(coreTypes.stringClass);
     _addInstantiatedExternalSubclass(coreTypes.intClass);
     _addInstantiatedExternalSubclass(coreTypes.boolClass);
     _addInstantiatedExternalSubclass(coreTypes.nullClass);
-    _addInstantiatedExternalSubclass(coreTypes.functionClass);
-    _addInstantiatedExternalSubclass(coreTypes.invocationClass);
   }
 
   /// Registers the given name as seen in a dynamic dispatch, and discovers used
   /// instance members accordingly.
-  void _addDispatchedName(Class receiver, Name name) {
-    int index = hierarchy.getClassIndex(receiver);
-    Set<Name> receiverNames = _dispatchedNames[index] ??= new Set<Name>();
+  void _addDispatchedName(Name name) {
     // TODO(asgerf): make use of selector arity and getter/setter kind
-    if (receiverNames.add(name)) {
-      List<TreeNode> candidates = _dispatchTargetCandidates[name];
-      if (candidates != null) {
-        for (int i = 0; i < candidates.length; i += 2) {
-          Class host = candidates[i];
-          if (hierarchy.isSubtypeOf(host, receiver)) {
-            // This (host, member) pair is a potential target of the dispatch.
-            Member member = candidates[i + 1];
-
-            // Remove the (host,member) pair from the candidate list.
-            // Move the last pair into the current index and shrink the list.
-            int lastPair = candidates.length - 2;
-            candidates[i] = candidates[lastPair];
-            candidates[i + 1] = candidates[lastPair + 1];
-            candidates.length -= 2;
-            i -= 2; // Revisit the same index now that it has been updated.
-
-            // Mark the pair as used.  This should be done after removing it
-            // from the candidate list, since this call may recursively scan
-            // for more used members.
-            _addUsedMember(host, member);
-          }
+    if (_dispatchedNames.add(name)) {
+      List<TreeNode> targets = _dispatchTargetCandidates[name];
+      if (targets != null) {
+        for (int i = 0; i < targets.length; i += 2) {
+          _addUsedMember(targets[i], targets[i + 1]);
         }
       }
-      var subtypes = hierarchy.getSubtypesOf(receiver);
-      var receiverSet = _receiversOfName[name];
-      _receiversOfName[name] = receiverSet == null
-          ? subtypes
-          : _receiversOfName[name].union(subtypes);
     }
   }
 
   /// Registers the given method as a potential target of dynamic dispatch on
   /// the given class.
   void _addDispatchTarget(Class host, Member member) {
-    ClassSet receivers = _receiversOfName[member.name];
-    if (receivers != null && receivers.contains(host)) {
+    if (_dispatchedNames.contains(member.name)) {
       _addUsedMember(host, member);
     } else {
       _dispatchTargetCandidates.putIfAbsent(member.name, _makeTreeNodeList)
         ..add(host)
         ..add(member);
     }
   }
 
   static List<TreeNode> _makeTreeNodeList() => <TreeNode>[];
 
@@ skipping 23 matching lines @@
     if (oldRetention.index >= ClassRetention.ExternalInstance.index) {
       return;
     }
     _propagateClassInstanceLevel(classNode, oldRetention);
     for (Member member in hierarchy.getInterfaceMembers(classNode)) {
       if (member is Field) {
         _covariantVisitor.visit(member.type);
       } else {
         _addCallToExternalProcedure(member);
       }
-      _addDispatchTarget(classNode, member);
-    }
-    for (Member member
-        in hierarchy.getInterfaceMembers(classNode, setters: true)) {
-      _addDispatchTarget(classNode, member);
     }
   }
 
   /// Called when the retention level for [classNode] has been raised from
   /// [oldRetention] to instance level.
   ///
   /// Ensures that the relevant members are put in the worklist, and super types
   /// and raised to hierarchy level.
   void _propagateClassInstanceLevel(
       Class classNode, ClassRetention oldRetention) {
@@ skipping 74 matching lines @@
     }
   }
 
   /// Registers the given member as being used, in the following sense:
   /// - Fields are used if they can be read or written or their initializer is
   ///   evaluated.
   /// - Constructors are used if they can be invoked, either directly or through
   ///   the initializer list of another constructor.
   /// - Procedures are used if they can be invoked or torn off.
   void _addUsedMember(Class host, Member member) {
-    if (member.enclosingLibrary == _mirrorsLibrary) {
-      throw new _UsingMirrorsException();
-    }
     if (host != null) {
       // Check if the member has been seen with this host before.
       int index = hierarchy.getClassIndex(host);
       Set<Member> members = _usedMembersWithHost[index] ??= new Set<Member>();
       if (!members.add(member)) return;
       _usedMembers.putIfAbsent(member, _makeIncompleteSummary);
     } else {
       // Check if the member has been seen before.
       if (_usedMembers.containsKey(member)) return;
       _usedMembers[member] = _makeIncompleteSummary();
@@ skipping 22 matching lines @@
     for (int i = 0; i < function.namedParameters.length; ++i) {
       _contravariantVisitor.visit(function.namedParameters[i].type);
     }
   }
 
   /// Called when external code may invoke the interface of the given class.
   void _addEscapedClass(Class node) {
     if (!_escapedClasses.add(node)) return;
     for (Member member in hierarchy.getInterfaceMembers(node)) {
       if (member is Procedure) {
-        _addDispatchedName(node, member.name);
+        _addDispatchedName(member.name);
       }
     }
   }
 
   /// Creates a incomplete summary object, indicating that a member has not
   /// yet been analyzed.
   static List<Node> _makeIncompleteSummary() => <Node>[null];
 
   bool isIncompleteSummary(List<Node> summary) {
     return summary.isNotEmpty && summary[0] == null;
@@ skipping 48 matching lines @@
 }
 
 /// Sentinel that occurs in method summaries in front of each name that should
 /// be interpreted as a setter.
 final Node _setterSentinel = const InvalidType();
 
 /// Searches the AST for static references and dynamically dispatched names.
 class _TreeShakerVisitor extends RecursiveVisitor {
   final TreeShaker shaker;
   final CoreTypes coreTypes;
-  final TypeEnvironment types;
-  final bool strongMode;
   List<Node> summary;
 
   _TreeShakerVisitor(TreeShaker shaker)
       : this.shaker = shaker,
-        this.coreTypes = shaker.coreTypes,
-        this.strongMode = shaker.strongMode,
-        this.types = new TypeEnvironment(shaker.coreTypes, shaker.hierarchy) {
-    types.errorHandler = handleError;
-  }
+        this.coreTypes = shaker.coreTypes;
 
-  void handleError(TreeNode node, String message) {
-    print('[error] $message (${node.location})');
-  }
-
-  void analyzeAndBuildSummary(Member member, List<Node> summary) {
+  void analyzeAndBuildSummary(Node node, List<Node> summary) {
     this.summary = summary;
-    types.thisType = member.enclosingClass?.thisType;
-    member.accept(this);
-  }
-
-  void visitMemberInterface(Member node) {
-    if (node is Field) {
-      node.type.accept(this);
-    } else if (node is Procedure) {
-      visitFunctionInterface(node.function);
-    }
-  }
-
-  visitFunctionInterface(FunctionNode node) {
-    for (var parameter in node.typeParameters) {
-      parameter.bound.accept(this);
-    }
-    for (var parameter in node.positionalParameters) {
-      parameter.type.accept(this);
-    }
-    for (var parameter in node.namedParameters) {
-      parameter.type.accept(this);
-    }
-    node.returnType.accept(this);
+    node.accept(this);
   }
 
   @override
   visitFunctionNode(FunctionNode node) {
     switch (node.asyncMarker) {
       case AsyncMarker.Sync:
         break;
       case AsyncMarker.SyncStar:
         shaker._addInstantiatedExternalSubclass(coreTypes.iterableClass);
         break;
@@ skipping 57 matching lines @@
   @override
   visitDirectMethodInvocation(DirectMethodInvocation node) {
     if (node.receiver is! ThisExpression) {
       // TODO(asgerf): Support arbitrary direct calls.
       throw 'Direct calls are only supported on "this"';
     }
     addUseFromCurrentHost(node.target);
     node.visitChildren(this);
   }
 
-  Class getKnownSupertype(DartType type) {
-    if (type is InterfaceType) {
-      return type.classNode;
-    } else if (type is TypeParameterType) {
-      return getKnownSupertype(type.parameter.bound);
-    } else if (type is FunctionType) {
-      return coreTypes.functionClass;
-    } else if (type is BottomType) {
-      return coreTypes.nullClass;
-    } else {
-      return coreTypes.objectClass;
-    }
-  }
-
-  Class getStaticType(Expression node) {
-    if (!strongMode) return coreTypes.objectClass;
-    return getKnownSupertype(node.getStaticType(types));
-  }
-
   @override
   visitMethodInvocation(MethodInvocation node) {
     if (node.receiver is ThisExpression) {
       addSelfDispatch(node.name);
     } else {
-      shaker._addDispatchedName(getStaticType(node.receiver), node.name);
-      if (node.interfaceTarget != null) {
-        shaker._typedCalls.add(node);
-      }
+      shaker._addDispatchedName(node.name);
     }
     node.visitChildren(this);
   }
 
   @override
   visitStaticGet(StaticGet node) {
     addStaticUse(node.target);
     node.visitChildren(this);
   }
 
@@ skipping 21 matching lines @@
     }
     addUseFromCurrentHost(node.target);
     node.visitChildren(this);
   }
 
   @override
   visitPropertyGet(PropertyGet node) {
     if (node.receiver is ThisExpression) {
       addSelfDispatch(node.name);
     } else {
-      shaker._addDispatchedName(getStaticType(node.receiver), node.name);
-      if (node.interfaceTarget != null) {
-        shaker._typedCalls.add(node);
-      }
+      shaker._addDispatchedName(node.name);
     }
     node.visitChildren(this);
   }
 
   @override
   visitPropertySet(PropertySet node) {
     if (node.receiver is ThisExpression) {
       addSelfDispatch(node.name, setter: true);
     } else {
-      shaker._addDispatchedName(getStaticType(node.receiver), node.name);
-      if (node.interfaceTarget != null) {
-        shaker._typedCalls.add(node);
-      }
+      shaker._addDispatchedName(node.name);
     }
     node.visitChildren(this);
   }
 
   @override
   visitListLiteral(ListLiteral node) {
     shaker._addInstantiatedExternalSubclass(coreTypes.listClass);
     node.visitChildren(this);
   }
 
   @override
   visitMapLiteral(MapLiteral node) {
     shaker._addInstantiatedExternalSubclass(coreTypes.mapClass);
     node.visitChildren(this);
   }
 
   static final Name _toStringName = new Name('toString');
 
   @override
   visitStringConcatenation(StringConcatenation node) {
-    for (var expression in node.expressions) {
-      shaker._addDispatchedName(getStaticType(expression), _toStringName);
-    }
+    shaker._addDispatchedName(_toStringName);
     node.visitChildren(this);
   }
 
   @override
   visitInterfaceType(InterfaceType node) {
     shaker._addClassUsedInType(node.classNode);
     node.visitChildren(this);
   }
 
   @override
-  visitSupertype(Supertype node) {
-    shaker._addClassUsedInType(node.classNode);
-    node.visitChildren(this);
-  }
-
-  @override
   visitDoubleLiteral(DoubleLiteral node) {
     shaker._addInstantiatedExternalSubclass(coreTypes.doubleClass);
   }
 
   @override
   visitSymbolLiteral(SymbolLiteral node) {
     shaker._addInstantiatedExternalSubclass(coreTypes.symbolClass);
+    // Note: we do not support 'dart:mirrors' right now, so nothing else needs
+    // to be done for symbols.
   }
 
   @override
   visitTypeLiteral(TypeLiteral node) {
     shaker._addInstantiatedExternalSubclass(coreTypes.typeClass);
     node.visitChildren(this);
   }
 }
 
 /// The degree to which a class is needed in a program.
@@ skipping 18 matching lines @@
 }
 
 /// Removes classes and members that are not needed.
 ///
 /// There must not be any dangling references in the program afterwards.
 class _TreeShakingTransformer extends Transformer {
   final TreeShaker shaker;
 
   _TreeShakingTransformer(this.shaker);
 
-  Member _translateInterfaceTarget(Member target) {
-    return target != null && shaker.isMemberUsed(target) ? target : null;
-  }
-
   void transform(Program program) {
     for (var library in program.libraries) {
       if (library.importUri.scheme == 'dart') {
-        // The backend expects certain things to be present in the core
-        // libraries, so we currently don't shake off anything there.
+        // As long as patching happens in the backend, we cannot shake off
+        // anything in the core libraries.
         continue;
       }
       library.transformChildren(this);
       // Note: we can't shake off empty libraries yet since we don't check if
       // there are private names that use the library.
     }
-    for (Expression node in shaker._typedCalls) {
-      // We should not leave dangling references, so if the target of a typed
-      // call has been removed, we must remove the reference.  The receiver of
-      // such a call can only be null.
-      // TODO(asgerf): Rewrite to a NSM call instead of adding dynamic calls.
-      if (node is MethodInvocation) {
-        node.interfaceTarget = _translateInterfaceTarget(node.interfaceTarget);
-      } else if (node is PropertyGet) {
-        node.interfaceTarget = _translateInterfaceTarget(node.interfaceTarget);
-      } else if (node is PropertySet) {
-        node.interfaceTarget = _translateInterfaceTarget(node.interfaceTarget);
-      }
-    }
   }
 
   Class visitClass(Class node) {
     switch (shaker.getClassRetention(node)) {
       case ClassRetention.None:
         return null; // Remove the class.
 
       case ClassRetention.Namespace:
         // The class is only a namespace for static members.  Remove its
         // hierarchy information.   This is mandatory, since these references
         // might otherwise become dangling.
         node.supertype = shaker.coreTypes.objectClass.asRawSupertype;
         node.implementedTypes.clear();
         node.typeParameters.clear();
-        node.isAbstract = true;
         // Mixin applications cannot have static members.
         assert(node.mixedInType == null);
         // Unused members will be removed below.
         break;
 
       case ClassRetention.Hierarchy:
-        node.isAbstract = true;
-        break;
-
       case ClassRetention.Instance:
       case ClassRetention.ExternalInstance:
         break;
     }
     node.transformChildren(this);
+    if (node.constructors.isEmpty && node.procedures.isEmpty) {
+      // The VM does not like classes without any members, so ensure there is
+      // always a constructor left.
+      node.addMember(new Constructor(new FunctionNode(new EmptyStatement())));
+    }
     return node;
   }
 
   Member defaultMember(Member node) {
-    if (!shaker.isMemberBodyUsed(node)) {
-      if (!shaker.isMemberOverridden(node)) {
-        return null;
-      }
-      if (node is Procedure) {
-        // Remove body of unused member.
-        if (node.enclosingClass.isAbstract) {
-          node.isAbstract = true;
-          node.function.body = null;
-        } else {
-          // If the enclosing class is not abstract, the method should still
-          // have a body even if it can never be called.
-          if (node.function.body != null) {
-            node.function.body = new ExpressionStatement(
-                new Throw(new StringLiteral('Method removed by tree-shaking')))
-              ..parent = node.function;
-          }
-        }
-        node.function.asyncMarker = AsyncMarker.Sync;
-      } else if (node is Field) {
-        node.initializer = null;
-      }
+    if (!shaker.isMemberUsed(node)) {
+      return null; // Remove unused member.
     }
     return node;
   }
 
   TreeNode defaultTreeNode(TreeNode node) {
     return node; // Do not traverse into other nodes.
   }
 }
 
 class _ExternalTypeVisitor extends DartTypeVisitor {
@@ skipping 13 matching lines @@
       type?.accept(this);
     } else if (isContravariant) {
       type?.accept(shaker._covariantVisitor);
     } else {
       type?.accept(shaker._contravariantVisitor);
     }
   }
 
   visitCovariant(DartType type) => type?.accept(this);
 
-  visitInvariant(DartType type) => shaker._invariantVisitor.visit(type);
+  visitBivariant(DartType type) => shaker._bivariantVisitor.visit(type);
 
   visitInvalidType(InvalidType node) {}
 
   visitDynamicType(DynamicType node) {
     // TODO(asgerf): Find a suitable model for untyped externals, e.g. track
     // them to the first type boundary.
   }
 
   visitVoidType(VoidType node) {}
 
   visitInterfaceType(InterfaceType node) {
     if (isCovariant) {
       shaker._addInstantiatedExternalSubclass(node.classNode);
     }
     if (isContravariant) {
       shaker._addEscapedClass(node.classNode);
     }
     for (int i = 0; i < node.typeArguments.length; ++i) {
       DartType typeArgument = node.typeArguments[i];
       // In practice we don't get much out of analyzing variance here, so
       // just use a whitelist of classes that can be seen as covariant
       // for external purposes.
       // TODO(asgerf): Variance analysis might pay off for other external APIs.
       if (isWhitelistedCovariant(node.classNode)) {
         visitCovariant(typeArgument);
       } else {
-        visitInvariant(typeArgument);
+        visitBivariant(typeArgument);
       }
     }
   }
 
   visitFunctionType(FunctionType node) {
     visit(node.returnType);
     for (int i = 0; i < node.positionalParameters.length; ++i) {
       visitContravariant(node.positionalParameters[i]);
     }
     for (int i = 0; i < node.namedParameters.length; ++i) {
       visitContravariant(node.namedParameters[i].type);
     }
   }
 
   visitTypeParameterType(TypeParameterType node) {}
 
   /// Just treat a couple of whitelisted classes as having covariant type
   /// parameters.
   bool isWhitelistedCovariant(Class classNode) {
     if (classNode.typeParameters.isEmpty) return false;
     CoreTypes coreTypes = shaker.coreTypes;
     return classNode == coreTypes.iteratorClass ||
         classNode == coreTypes.iterableClass ||
         classNode == coreTypes.futureClass ||
         classNode == coreTypes.streamClass ||
         classNode == coreTypes.listClass ||
         classNode == coreTypes.mapClass;
   }
 }
-
-/// Exception that is thrown to stop the tree shaking analysis when a use
-/// of `dart:mirrors` is found.
-class _UsingMirrorsException {}