// Copyright (c) 2014, 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.

/// Defines the element model. The element model describes the semantic (as
/// opposed to syntactic) structure of Dart code. The syntactic structure of the
/// code is modeled by the [AST
/// structure](../analyzer.dart.ast.ast/analyzer.dart.ast.ast-library.html).
///
/// The element model consists of two closely related kinds of objects: elements
/// (instances of a subclass of [Element]) and types. This library defines the
/// elements, the types are defined in
/// [type.dart](../dart_element_type/dart_element_type-library.html).
///
/// Generally speaking, an element represents something that is declared in the
/// code, such as a class, method, or variable. Elements are organized in a tree
/// structure in which the children of an element are the elements that are
/// logically (and often syntactically) part of the declaration of the parent.
/// For example, the elements representing the methods and fields in a class are
/// children of the element representing the class.
///
/// Every complete element structure is rooted by an instance of the class
/// [LibraryElement]. A library element represents a single Dart library. Every
/// library is defined by one or more compilation units (the library and all of
/// its parts). The compilation units are represented by the class
/// [CompilationUnitElement] and are children of the library that is defined by
/// them. Each compilation unit can contain zero or more top-level declarations,
/// such as classes, functions, and variables. Each of these is in turn
/// represented as an element that is a child of the compilation unit. Classes
/// contain methods and fields, methods can contain local variables, etc.
///
/// The element model does not contain everything in the code, only those things
/// that are declared by the code. For example, it does not include any
/// representation of the statements in a method body, but if one of those
/// statements declares a local variable then the local variable will be
/// represented by an element.
import 'package:analyzer/dart/analysis/session.dart';
import 'package:analyzer/dart/constant/value.dart';
import 'package:analyzer/dart/element/nullability_suffix.dart';
import 'package:analyzer/dart/element/type.dart';
import 'package:analyzer/dart/element/type_provider.dart';
import 'package:analyzer/dart/element/type_system.dart';
import 'package:analyzer/error/error.dart';
import 'package:analyzer/src/dart/constant/evaluation.dart';
import 'package:analyzer/src/dart/resolver/scope.dart' show Namespace;
import 'package:analyzer/src/generated/engine.dart' show AnalysisContext;
import 'package:analyzer/src/generated/java_engine.dart';
import 'package:analyzer/src/generated/source.dart';
import 'package:analyzer/src/generated/utilities_dart.dart';
import 'package:analyzer/src/task/api/model.dart' show AnalysisTarget;
import 'package:meta/meta.dart';
import 'package:pub_semver/pub_semver.dart';

/// An element that represents a class or a mixin. The class can be defined by
/// either a class declaration (with a class body), a mixin application (without
/// a class body), a mixin declaration, or an enum declaration.
///
/// Clients may not extend, implement or mix-in this class.
abstract class ClassElement
    implements TypeDefiningElement, TypeParameterizedElement {
  /// Return a list containing all of the accessors (getters and setters)
  /// declared in this class.
  List<PropertyAccessorElement> get accessors;

  /// Return a list containing all the supertypes defined for this class and its
  /// supertypes. This includes superclasses, mixins, interfaces and superclass
  /// constraints.
  List<InterfaceType> get allSupertypes;

  /// Return a list containing all of the constructors declared in this class.
  /// The list will be empty if there are no constructors defined for this
  /// class, as is the case when this element represents an enum or a mixin.
  List<ConstructorElement> get constructors;

  /// Return a list containing all of the fields declared in this class.
  List<FieldElement> get fields;

  /// Return `true` if this class or its superclass declares a non-final
  /// instance field.
  bool get hasNonFinalField;

  /// Return `true` if this class has at least one reference to `super` (and
  /// hence cannot be used as a mixin), or `false` if this element represents a
  /// mixin, even if the mixin has a reference to `super`, because it is allowed
  /// to be used as a mixin.
  @Deprecated('It was used internally, should not be part of API')
  bool get hasReferenceToSuper;

  /// Return `true` if this class declares a static member.
  bool get hasStaticMember;

  /// Return a list containing all of the interfaces that are implemented by
  /// this class.
  ///
  /// <b>Note:</b> Because the element model represents the state of the code,
  /// it is possible for it to be semantically invalid. In particular, it is not
  /// safe to assume that the inheritance structure of a class does not contain
  /// a cycle. Clients that traverse the inheritance structure must explicitly
  /// guard against infinite loops.
  List<InterfaceType> get interfaces;

  /// Return `true` if this class is abstract. A class is abstract if it has an
  /// explicit `abstract` modifier or if it is implicitly abstract, such as a
  /// class defined by a mixin declaration. Note, that this definition of
  /// <i>abstract</i> is different from <i>has unimplemented members</i>.
  bool get isAbstract;

  /// Return `true` if this class represents the class 'Object' defined in the
  /// dart:core library.
  bool get isDartCoreObject;

  /// Return `true` if this class is defined by an enum declaration.
  bool get isEnum;

  /// Return `true` if this class is defined by a mixin declaration.
  bool get isMixin;

  /// Return `true` if this class is a mixin application.  A class is a mixin
  /// application if it was declared using the syntax "class A = B with C;".
  bool get isMixinApplication;

  /// Return `true` if this class [isProxy], or if it inherits the proxy
  /// annotation from a supertype.
  @Deprecated(
    'The @proxy annotation is deprecated in the langauge, and will be removed',
  )
  bool get isOrInheritsProxy;

  /// Return `true` if this element has an annotation of the form '@proxy'.
  @Deprecated(
    'The @proxy annotation is deprecated in the langauge, and will be removed',
  )
  bool get isProxy;

  /// Return `true` if this class can validly be used as a mixin when defining
  /// another class. For classes defined by a mixin declaration, the result is
  /// always `true`. For classes defined by a class declaration or a mixin
  /// application, the behavior of this method is defined by the Dart Language
  /// Specification in section 9:
  /// <blockquote>
  /// It is a compile-time error if a declared or derived mixin refers to super.
  /// It is a compile-time error if a declared or derived mixin explicitly
  /// declares a constructor. It is a compile-time error if a mixin is derived
  /// from a class whose superclass is not Object.
  /// </blockquote>
  bool get isValidMixin;

  /// Return a list containing all of the methods declared in this class.
  List<MethodElement> get methods;

  /// Return a list containing all of the mixins that are applied to the class
  /// being extended in order to derive the superclass of this class.
  ///
  /// <b>Note:</b> Because the element model represents the state of the code,
  /// it is possible for it to be semantically invalid. In particular, it is not
  /// safe to assume that the inheritance structure of a class does not contain
  /// a cycle. Clients that traverse the inheritance structure must explicitly
  /// guard against infinite loops.
  List<InterfaceType> get mixins;

  /// Return a list containing all of the superclass constraints defined for
  /// this class. The list will be empty if this class does not represent a
  /// mixin declaration. If this class _does_ represent a mixin declaration but
  /// the declaration does not have an `on` clause, then the list will contain
  /// the type for the class `Object`.
  ///
  /// <b>Note:</b> Because the element model represents the state of the code,
  /// it is possible for it to be semantically invalid. In particular, it is not
  /// safe to assume that the inheritance structure of a class does not contain
  /// a cycle. Clients that traverse the inheritance structure must explicitly
  /// guard against infinite loops.
  List<InterfaceType> get superclassConstraints;

  /// Return the superclass of this class, or `null` if either the class
  /// represents the class 'Object' or if the class represents a mixin
  /// declaration. All other classes will have a non-`null` superclass. If the
  /// superclass was not explicitly declared then the implicit superclass
  /// 'Object' will be returned.
  ///
  /// <b>Note:</b> Because the element model represents the state of the code,
  /// it is possible for it to be semantically invalid. In particular, it is not
  /// safe to assume that the inheritance structure of a class does not contain
  /// a cycle. Clients that traverse the inheritance structure must explicitly
  /// guard against infinite loops.
  InterfaceType get supertype;

  /// Return the type of `this` expression for this class.
  ///
  /// For a class like `class MyClass<T, U> {}` the returned type is equivalent
  /// to the type `MyClass<T, U>`. So, the type arguments are the types of the
  /// type parameters, and either `none` or `star` nullability suffix is used
  /// for the type arguments, and the returned type depending on the
  /// nullability status of the declaring library.
  InterfaceType get thisType;

  @override
  @deprecated
  InterfaceType get type;

  /// Return the unnamed constructor declared in this class, or `null` if either
  /// this class does not declare an unnamed constructor but does declare named
  /// constructors or if this class represents a mixin declaration. The returned
  /// constructor will be synthetic if this class does not declare any
  /// constructors, in which case it will represent the default constructor for
  /// the class.
  ConstructorElement get unnamedConstructor;

  /// Return the field (synthetic or explicit) defined in this class that has
  /// the given [name], or `null` if this class does not define a field with the
  /// given name.
  FieldElement getField(String name);

  /// Return the element representing the getter with the given [name] that is
  /// declared in this class, or `null` if this class does not declare a getter
  /// with the given name.
  PropertyAccessorElement getGetter(String name);

  /// Return the element representing the method with the given [name] that is
  /// declared in this class, or `null` if this class does not declare a method
  /// with the given name.
  MethodElement getMethod(String name);

  /// Return the named constructor declared in this class with the given [name],
  /// or `null` if this class does not declare a named constructor with the
  /// given name.
  ConstructorElement getNamedConstructor(String name);

  /// Return the element representing the setter with the given [name] that is
  /// declared in this class, or `null` if this class does not declare a setter
  /// with the given name.
  PropertyAccessorElement getSetter(String name);

  /// Create the [InterfaceType] for this class with the given [typeArguments]
  /// and [nullabilitySuffix].
  InterfaceType instantiate({
    @required List<DartType> typeArguments,
    @required NullabilitySuffix nullabilitySuffix,
  });

  /// Create the [InterfaceType] for this class with type arguments that
  /// correspond to the bounds of the type parameters, and the given
  /// [nullabilitySuffix].
  @Deprecated('Use TypeSystem.instantiateToBounds2() instead')
  InterfaceType instantiateToBounds({
    @required NullabilitySuffix nullabilitySuffix,
  });

  /// Return the element representing the method that results from looking up
  /// the given [methodName] in this class with respect to the given [library],
  /// ignoring abstract methods, or `null` if the look up fails. The behavior of
  /// this method is defined by the Dart Language Specification in section
  /// 16.15.1:
  /// <blockquote>
  /// The result of looking up method <i>m</i> in class <i>C</i> with respect to
  /// library <i>L</i> is: If <i>C</i> declares an instance method named
  /// <i>m</i> that is accessible to <i>L</i>, then that method is the result of
  /// the lookup. Otherwise, if <i>C</i> has a superclass <i>S</i>, then the
  /// result of the lookup is the result of looking up method <i>m</i> in
  /// <i>S</i> with respect to <i>L</i>. Otherwise, we say that the lookup has
  /// failed.
  /// </blockquote>
  MethodElement lookUpConcreteMethod(String methodName, LibraryElement library);

  /// Return the element representing the getter that results from looking up
  /// the given [getterName] in this class with respect to the given [library],
  /// or `null` if the look up fails. The behavior of this method is defined by
  /// the Dart Language Specification in section 16.15.2:
  /// <blockquote>
  /// The result of looking up getter (respectively setter) <i>m</i> in class
  /// <i>C</i> with respect to library <i>L</i> is: If <i>C</i> declares an
  /// instance getter (respectively setter) named <i>m</i> that is accessible to
  /// <i>L</i>, then that getter (respectively setter) is the result of the
  /// lookup. Otherwise, if <i>C</i> has a superclass <i>S</i>, then the result
  /// of the lookup is the result of looking up getter (respectively setter)
  /// <i>m</i> in <i>S</i> with respect to <i>L</i>. Otherwise, we say that the
  /// lookup has failed.
  /// </blockquote>
  PropertyAccessorElement lookUpGetter(
      String getterName, LibraryElement library);

  /// Return the element representing the getter that results from looking up
  /// the given [getterName] in the superclass of this class with respect to the
  /// given [library], ignoring abstract getters, or `null` if the look up
  /// fails.  The behavior of this method is defined by the Dart Language
  /// Specification in section 16.15.2:
  /// <blockquote>
  /// The result of looking up getter (respectively setter) <i>m</i> in class
  /// <i>C</i> with respect to library <i>L</i> is: If <i>C</i> declares an
  /// instance getter (respectively setter) named <i>m</i> that is accessible to
  /// <i>L</i>, then that getter (respectively setter) is the result of the
  /// lookup. Otherwise, if <i>C</i> has a superclass <i>S</i>, then the result
  /// of the lookup is the result of looking up getter (respectively setter)
  /// <i>m</i> in <i>S</i> with respect to <i>L</i>. Otherwise, we say that the
  /// lookup has failed.
  /// </blockquote>
  PropertyAccessorElement lookUpInheritedConcreteGetter(
      String getterName, LibraryElement library);

  /// Return the element representing the method that results from looking up
  /// the given [methodName] in the superclass of this class with respect to the
  /// given [library], ignoring abstract methods, or `null` if the look up
  /// fails.  The behavior of this method is defined by the Dart Language
  /// Specification in section 16.15.1:
  /// <blockquote>
  /// The result of looking up method <i>m</i> in class <i>C</i> with respect to
  /// library <i>L</i> is:  If <i>C</i> declares an instance method named
  /// <i>m</i> that is accessible to <i>L</i>, then that method is the result of
  /// the lookup. Otherwise, if <i>C</i> has a superclass <i>S</i>, then the
  /// result of the lookup is the result of looking up method <i>m</i> in
  /// <i>S</i> with respect to <i>L</i>. Otherwise, we say that the lookup has
  /// failed.
  /// </blockquote>
  MethodElement lookUpInheritedConcreteMethod(
      String methodName, LibraryElement library);

  /// Return the element representing the setter that results from looking up
  /// the given [setterName] in the superclass of this class with respect to the
  /// given [library], ignoring abstract setters, or `null` if the look up
  /// fails.  The behavior of this method is defined by the Dart Language
  /// Specification in section 16.15.2:
  /// <blockquote>
  /// The result of looking up getter (respectively setter) <i>m</i> in class
  /// <i>C</i> with respect to library <i>L</i> is:  If <i>C</i> declares an
  /// instance getter (respectively setter) named <i>m</i> that is accessible to
  /// <i>L</i>, then that getter (respectively setter) is the result of the
  /// lookup. Otherwise, if <i>C</i> has a superclass <i>S</i>, then the result
  /// of the lookup is the result of looking up getter (respectively setter)
  /// <i>m</i> in <i>S</i> with respect to <i>L</i>. Otherwise, we say that the
  /// lookup has failed.
  /// </blockquote>
  PropertyAccessorElement lookUpInheritedConcreteSetter(
      String setterName, LibraryElement library);

  /// Return the element representing the method that results from looking up
  /// the given [methodName] in the superclass of this class with respect to the
  /// given [library], or `null` if the look up fails. The behavior of this
  /// method is defined by the Dart Language Specification in section 16.15.1:
  /// <blockquote>
  /// The result of looking up method <i>m</i> in class <i>C</i> with respect to
  /// library <i>L</i> is:  If <i>C</i> declares an instance method named
  /// <i>m</i> that is accessible to <i>L</i>, then that method is the result of
  /// the lookup. Otherwise, if <i>C</i> has a superclass <i>S</i>, then the
  /// result of the lookup is the result of looking up method <i>m</i> in
  /// <i>S</i> with respect to <i>L</i>. Otherwise, we say that the lookup has
  /// failed.
  /// </blockquote>
  MethodElement lookUpInheritedMethod(
      String methodName, LibraryElement library);

  /// Return the element representing the method that results from looking up
  /// the given [methodName] in this class with respect to the given [library],
  /// or `null` if the look up fails. The behavior of this method is defined by
  /// the Dart Language Specification in section 16.15.1:
  /// <blockquote>
  /// The result of looking up method <i>m</i> in class <i>C</i> with respect to
  /// library <i>L</i> is:  If <i>C</i> declares an instance method named
  /// <i>m</i> that is accessible to <i>L</i>, then that method is the result of
  /// the lookup. Otherwise, if <i>C</i> has a superclass <i>S</i>, then the
  /// result of the lookup is the result of looking up method <i>m</i> in
  /// <i>S</i> with respect to <i>L</i>. Otherwise, we say that the lookup has
  /// failed.
  /// </blockquote>
  MethodElement lookUpMethod(String methodName, LibraryElement library);

  /// Return the element representing the setter that results from looking up
  /// the given [setterName] in this class with respect to the given [library],
  /// or `null` if the look up fails. The behavior of this method is defined by
  /// the Dart Language Specification in section 16.15.2:
  /// <blockquote>
  /// The result of looking up getter (respectively setter) <i>m</i> in class
  /// <i>C</i> with respect to library <i>L</i> is: If <i>C</i> declares an
  /// instance getter (respectively setter) named <i>m</i> that is accessible to
  /// <i>L</i>, then that getter (respectively setter) is the result of the
  /// lookup. Otherwise, if <i>C</i> has a superclass <i>S</i>, then the result
  /// of the lookup is the result of looking up getter (respectively setter)
  /// <i>m</i> in <i>S</i> with respect to <i>L</i>. Otherwise, we say that the
  /// lookup has failed.
  /// </blockquote>
  PropertyAccessorElement lookUpSetter(
      String setterName, LibraryElement library);
}

/// An element that is contained within a [ClassElement].
///
/// When the 'extension-methods' experiment is enabled, these elements can also
/// be contained within an extension element.
///
/// Clients may not extend, implement or mix-in this class.
abstract class ClassMemberElement implements Element {
  // TODO(brianwilkerson) Either remove this class or rename it to something
  //  more correct.

  /// Return `true` if this element is a static element. A static element is an
  /// element that is not associated with a particular instance, but rather with
  /// an entire library or class.
  bool get isStatic;
}

/// An element representing a compilation unit.
///
/// Clients may not extend, implement or mix-in this class.
abstract class CompilationUnitElement implements Element, UriReferencedElement {
  /// Return a list containing all of the top-level accessors (getters and
  /// setters) contained in this compilation unit.
  List<PropertyAccessorElement> get accessors;

  @override
  LibraryElement get enclosingElement;

  /// Return a list containing all of the enums contained in this compilation
  /// unit.
  List<ClassElement> get enums;

  /// Return a list containing all of the extensions contained in this
  /// compilation unit.
  List<ExtensionElement> get extensions;

  /// Return a list containing all of the top-level functions contained in this
  /// compilation unit.
  List<FunctionElement> get functions;

  /// Return a list containing all of the function type aliases contained in
  /// this compilation unit.
  List<FunctionTypeAliasElement> get functionTypeAliases;

  /// Return `true` if this compilation unit defines a top-level function named
  /// `loadLibrary`.
  bool get hasLoadLibraryFunction;

  /// Return the [LineInfo] for the [source], or `null` if not computed yet.
  LineInfo get lineInfo;

  /// Return a list containing all of the mixins contained in this compilation
  /// unit.
  List<ClassElement> get mixins;

  /// Return a list containing all of the top-level variables contained in this
  /// compilation unit.
  List<TopLevelVariableElement> get topLevelVariables;

  /// Return a list containing all of the classes contained in this compilation
  /// unit.
  List<ClassElement> get types;

  /// Return the enum defined in this compilation unit that has the given
  /// [name], or `null` if this compilation unit does not define an enum with
  /// the given name.
  ClassElement getEnum(String name);

  /// Return the class defined in this compilation unit that has the given
  /// [name], or `null` if this compilation unit does not define a class with
  /// the given name.
  ClassElement getType(String name);
}

/// An element representing a constructor or a factory method defined within a
/// class.
///
/// Clients may not extend, implement or mix-in this class.
abstract class ConstructorElement
    implements ClassMemberElement, ExecutableElement, ConstantEvaluationTarget {
  @override
  ConstructorElement get declaration;

  @override
  ClassElement get enclosingElement;

  /// Return `true` if this constructor is a const constructor.
  bool get isConst;

  /// Return `true` if this constructor can be used as a default constructor -
  /// unnamed and has no required parameters.
  bool get isDefaultConstructor;

  /// Return `true` if this constructor represents a factory constructor.
  bool get isFactory;

  /// Return the offset of the character immediately following the last
  /// character of this constructor's name, or `null` if not named.
  int get nameEnd;

  /// Return the offset of the `.` before this constructor name, or `null` if
  /// not named.
  int get periodOffset;

  /// Return the constructor to which this constructor is redirecting, or `null`
  /// if this constructor does not redirect to another constructor or if the
  /// library containing this constructor has not yet been resolved.
  ConstructorElement get redirectedConstructor;
}

/// The base class for all of the elements in the element model. Generally
/// speaking, the element model is a semantic model of the program that
/// represents things that are declared with a name and hence can be referenced
/// elsewhere in the code.
///
/// There are two exceptions to the general case. First, there are elements in
/// the element model that are created for the convenience of various kinds of
/// analysis but that do not have any corresponding declaration within the
/// source code. Such elements are marked as being <i>synthetic</i>. Examples of
/// synthetic elements include
/// * default constructors in classes that do not define any explicit
///   constructors,
/// * getters and setters that are induced by explicit field declarations,
/// * fields that are induced by explicit declarations of getters and setters,
///   and
/// * functions representing the initialization expression for a variable.
///
/// Second, there are elements in the element model that do not have a name.
/// These correspond to unnamed functions and exist in order to more accurately
/// represent the semantic structure of the program.
///
/// Clients may not extend, implement or mix-in this class.
abstract class Element implements AnalysisTarget {
  /// A comparator that can be used to sort elements by their name offset.
  /// Elements with a smaller offset will be sorted to be before elements with a
  /// larger name offset.
  static final Comparator<Element> SORT_BY_OFFSET =
      (Element firstElement, Element secondElement) =>
          firstElement.nameOffset - secondElement.nameOffset;

  /// Return the analysis context in which this element is defined.
  AnalysisContext get context;

  /// Return the declaration of this element. If the element is a view on an
  /// element, e.g. a method from an interface type, with substituted type
  /// parameters, return the corresponding element from the class, without any
  /// substitutions. If this element is already a declaration (or a synthetic
  /// element, e.g. a synthetic property accessor), return itself.
  Element get declaration;

  /// Return the display name of this element, or `null` if this element does
  /// not have a name.
  ///
  /// In most cases the name and the display name are the same. Differences
  /// though are cases such as setters where the name of some setter `set f(x)`
  /// is `f=`, instead of `f`.
  String get displayName;

  /// Return the content of the documentation comment (including delimiters) for
  /// this element, or `null` if this element does not or cannot have
  /// documentation.
  String get documentationComment;

  /// Return the element that either physically or logically encloses this
  /// element. This will be `null` if this element is a library because
  /// libraries are the top-level elements in the model.
  Element get enclosingElement;

  /// Return `true` if this element has an annotation of the form
  /// `@alwaysThrows`.
  bool get hasAlwaysThrows;

  /// Return `true` if this element has an annotation of the form `@deprecated`
  /// or `@Deprecated('..')`.
  bool get hasDeprecated;

  /// Return `true` if this element has an annotation of the form `@factory`.
  bool get hasFactory;

  /// Return `true` if this element has an annotation of the form `@isTest`.
  bool get hasIsTest;

  /// Return `true` if this element has an annotation of the form
  /// `@isTestGroup`.
  bool get hasIsTestGroup;

  /// Return `true` if this element has an annotation of the form `@JS(..)`.
  bool get hasJS;

  /// Return `true` if this element has an annotation of the form `@literal`.
  bool get hasLiteral;

  /// Return  `true` if this element has an annotation of the form `@mustCallSuper`.
  bool get hasMustCallSuper;

  /// Return  `true` if this element has an annotation of the form `@nonVirtual`.
  bool get hasNonVirtual;

  /// Return `true` if this element has an annotation of the form
  /// `@optionalTypeArgs`.
  bool get hasOptionalTypeArgs;

  /// Return `true` if this element has an annotation of the form `@override`.
  bool get hasOverride;

  /// Return `true` if this element has an annotation of the form `@protected`.
  bool get hasProtected;

  /// Return `true` if this element has an annotation of the form `@required`.
  bool get hasRequired;

  /// Return `true` if this element has an annotation of the form `@sealed`.
  bool get hasSealed;

  /// Return `true` if this element has an annotation of the form
  /// `@visibleForTemplate`.
  bool get hasVisibleForTemplate;

  /// Return `true` if this element has an annotation of the form
  /// `@visibleForTesting`.
  bool get hasVisibleForTesting;

  /// The unique integer identifier of this element.
  int get id;

  /// Return `true` if this element is private. Private elements are visible
  /// only within the library in which they are declared.
  bool get isPrivate;

  /// Return `true` if this element is public. Public elements are visible
  /// within any library that imports the library in which they are declared.
  bool get isPublic;

  /// Return `true` if this element is synthetic. A synthetic element is an
  /// element that is not represented in the source code explicitly, but is
  /// implied by the source code, such as the default constructor for a class
  /// that does not explicitly define any constructors.
  bool get isSynthetic;

  /// Return the kind of element that this is.
  ElementKind get kind;

  /// Return the library that contains this element. This will be the element
  /// itself if it is a library element. This will be `null` if this element is
  /// an HTML file because HTML files are not contained in libraries.
  LibraryElement get library;

  /// Return an object representing the location of this element in the element
  /// model. The object can be used to locate this element at a later time.
  ElementLocation get location;

  /// Return a list containing all of the metadata associated with this element.
  /// The array will be empty if the element does not have any metadata or if
  /// the library containing this element has not yet been resolved.
  List<ElementAnnotation> get metadata;

  /// Return the name of this element, or `null` if this element does not have a
  /// name.
  String get name;

  /// Return the length of the name of this element in the file that contains
  /// the declaration of this element, or `0` if this element does not have a
  /// name.
  int get nameLength;

  /// Return the offset of the name of this element in the file that contains
  /// the declaration of this element, or `-1` if this element is synthetic,
  /// does not have a name, or otherwise does not have an offset.
  int get nameOffset;

  /// Return the analysis session in which this element is defined.
  AnalysisSession get session;

  @override
  Source get source;

  /// Use the given [visitor] to visit this element. Return the value returned
  /// by the visitor as a result of visiting this element.
  T accept<T>(ElementVisitor<T> visitor);

  /// Return the most immediate ancestor of this element for which the
  /// [predicate] returns `true`, or `null` if there is no such ancestor. Note
  /// that this element will never be returned.
  @Deprecated('Use either thisOrAncestorMatching or thisOrAncestorOfType')
  E getAncestor<E extends Element>(Predicate<Element> predicate);

  /// Return the presentation of this element as it should appear when
  /// presented to users.
  ///
  /// If [withNullability] is `true`, then [NullabilitySuffix.question] and
  /// [NullabilitySuffix.star] in types will be be represented as `?` and `*`.
  /// [NullabilitySuffix.none] does not have any explicit presentation.
  ///
  /// If [withNullability] is `false`, nullability suffixes will not be
  /// included into the presentation.
  ///
  /// Clients should not depend on the content of the returned value as it will
  /// be changed if doing so would improve the UX.
  String getDisplayString({@required bool withNullability});

  /// Return a display name for the given element that includes the path to the
  /// compilation unit in which the type is defined. If [shortName] is `null`
  /// then [displayName] will be used as the name of this element. Otherwise
  /// the provided name will be used.
  // TODO(brianwilkerson) Make the parameter optional.
  String getExtendedDisplayName(String shortName);

  /// Return `true` if this element, assuming that it is within scope, is
  /// accessible to code in the given [library]. This is defined by the Dart
  /// Language Specification in section 3.2:
  /// <blockquote>
  /// A declaration <i>m</i> is accessible to library <i>L</i> if <i>m</i> is
  /// declared in <i>L</i> or if <i>m</i> is public.
  /// </blockquote>
  bool isAccessibleIn(LibraryElement library);

  /// Return either this element or the most immediate ancestor of this element
  /// for which the [predicate] returns `true`, or `null` if there is no such
  /// element.
  E thisOrAncestorMatching<E extends Element>(Predicate<Element> predicate);

  /// Return either this element or the most immediate ancestor of this element
  /// that has the given type, or `null` if there is no such element.
  E thisOrAncestorOfType<E extends Element>();

  /// Use the given [visitor] to visit all of the children of this element.
  /// There is no guarantee of the order in which the children will be visited.
  void visitChildren(ElementVisitor visitor);
}

/// A single annotation associated with an element.
///
/// Clients may not extend, implement or mix-in this class.
abstract class ElementAnnotation implements ConstantEvaluationTarget {
  /// Return the errors that were produced while computing a value for this
  /// annotation, or `null` if no value has been computed. If a value has been
  /// produced but no errors were generated, then the list will be empty.
  List<AnalysisError> get constantEvaluationErrors;

  /// Return a representation of the value of this annotation, or `null` if the
  /// value of this annotation has not been computed or if the value could not
  /// be computed because of errors.
  DartObject get constantValue;

  /// Return the element representing the field, variable, or const constructor
  /// being used as an annotation.
  Element get element;

  /// Return `true` if this annotation marks the associated function as always
  /// throwing.
  bool get isAlwaysThrows;

  /// Return `true` if this annotation marks the associated element as being
  /// deprecated.
  bool get isDeprecated;

  /// Return `true` if this annotation marks the associated member as a factory.
  bool get isFactory;

  /// Return `true` if this annotation marks the associated class and its
  /// subclasses as being immutable.
  bool get isImmutable;

  /// Return `true` if this annotation marks the associated member as running
  /// a single test.
  bool get isIsTest;

  /// Return `true` if this annotation marks the associated member as running
  /// a test group.
  bool get isIsTestGroup;

  /// Return `true` if this annotation marks the associated element with the
  /// `JS` annotation.
  bool get isJS;

  /// Return `true` if this annotation marks the associated constructor as
  /// being literal.
  bool get isLiteral;

  /// Return `true` if this annotation marks the associated member as requiring
  /// overriding methods to call super.
  bool get isMustCallSuper;

  /// Return `true` if this annotation marks the associated member as being
  /// non-virtual.
  bool get isNonVirtual;

  /// Return `true` if this annotation marks the associated type as
  /// having "optional" type arguments.
  bool get isOptionalTypeArgs;

  /// Return `true` if this annotation marks the associated method as being
  /// expected to override an inherited method.
  bool get isOverride;

  /// Return `true` if this annotation marks the associated member as being
  /// protected.
  bool get isProtected;

  /// Return `true` if this annotation marks the associated class as
  /// implementing a proxy object.
  bool get isProxy;

  /// Return `true` if this annotation marks the associated member as being
  /// required.
  bool get isRequired;

  /// Return `true` if this annotation marks the associated class as being
  /// sealed.
  bool get isSealed;

  /// Return `true` if this annotation marks the associated member as being
  /// visible for template files.
  bool get isVisibleForTemplate;

  /// Return `true` if this annotation marks the associated member as being
  /// visible for testing.
  bool get isVisibleForTesting;

  /// Return a representation of the value of this annotation, forcing the value
  /// to be computed if it had not previously been computed, or `null` if the
  /// value of this annotation could not be computed because of errors.
  DartObject computeConstantValue();

  /// Return a textual description of this annotation in a form approximating
  /// valid source. The returned string will not be valid source primarily in
  /// the case where the annotation itself is not well-formed.
  String toSource();
}

/// The kind of elements in the element model.
///
/// Clients may not extend, implement or mix-in this class.
class ElementKind implements Comparable<ElementKind> {
  static const ElementKind CLASS = ElementKind('CLASS', 0, "class");

  static const ElementKind COMPILATION_UNIT =
      ElementKind('COMPILATION_UNIT', 1, "compilation unit");

  static const ElementKind CONSTRUCTOR =
      ElementKind('CONSTRUCTOR', 2, "constructor");

  static const ElementKind DYNAMIC = ElementKind('DYNAMIC', 3, "<dynamic>");

  static const ElementKind ENUM = ElementKind('ENUM', 4, "enum");

  static const ElementKind ERROR = ElementKind('ERROR', 5, "<error>");

  static const ElementKind EXPORT =
      ElementKind('EXPORT', 6, "export directive");

  static const ElementKind EXTENSION = ElementKind('EXTENSION', 7, "extension");

  static const ElementKind FIELD = ElementKind('FIELD', 8, "field");

  static const ElementKind FUNCTION = ElementKind('FUNCTION', 9, "function");

  static const ElementKind GENERIC_FUNCTION_TYPE =
      ElementKind('GENERIC_FUNCTION_TYPE', 10, 'generic function type');

  static const ElementKind GETTER = ElementKind('GETTER', 11, "getter");

  static const ElementKind IMPORT =
      ElementKind('IMPORT', 12, "import directive");

  static const ElementKind LABEL = ElementKind('LABEL', 13, "label");

  static const ElementKind LIBRARY = ElementKind('LIBRARY', 14, "library");

  static const ElementKind LOCAL_VARIABLE =
      ElementKind('LOCAL_VARIABLE', 15, "local variable");

  static const ElementKind METHOD = ElementKind('METHOD', 16, "method");

  static const ElementKind NAME = ElementKind('NAME', 17, "<name>");

  static const ElementKind NEVER = ElementKind('NEVER', 18, "<never>");

  static const ElementKind PARAMETER =
      ElementKind('PARAMETER', 19, "parameter");

  static const ElementKind PREFIX = ElementKind('PREFIX', 20, "import prefix");

  static const ElementKind SETTER = ElementKind('SETTER', 21, "setter");

  static const ElementKind TOP_LEVEL_VARIABLE =
      ElementKind('TOP_LEVEL_VARIABLE', 22, "top level variable");

  static const ElementKind FUNCTION_TYPE_ALIAS =
      ElementKind('FUNCTION_TYPE_ALIAS', 23, "function type alias");

  static const ElementKind TYPE_PARAMETER =
      ElementKind('TYPE_PARAMETER', 24, "type parameter");

  static const ElementKind UNIVERSE = ElementKind('UNIVERSE', 25, "<universe>");

  static const List<ElementKind> values = [
    CLASS,
    COMPILATION_UNIT,
    CONSTRUCTOR,
    DYNAMIC,
    ENUM,
    ERROR,
    EXPORT,
    FIELD,
    FUNCTION,
    GENERIC_FUNCTION_TYPE,
    GETTER,
    IMPORT,
    LABEL,
    LIBRARY,
    LOCAL_VARIABLE,
    METHOD,
    NAME,
    NEVER,
    PARAMETER,
    PREFIX,
    SETTER,
    TOP_LEVEL_VARIABLE,
    FUNCTION_TYPE_ALIAS,
    TYPE_PARAMETER,
    UNIVERSE
  ];

  /// The name of this element kind.
  final String name;

  /// The ordinal value of the element kind.
  final int ordinal;

  /// The name displayed in the UI for this kind of element.
  final String displayName;

  /// Initialize a newly created element kind to have the given [displayName].
  const ElementKind(this.name, this.ordinal, this.displayName);

  @override
  int get hashCode => ordinal;

  @override
  int compareTo(ElementKind other) => ordinal - other.ordinal;

  @override
  String toString() => name;

  /// Return the kind of the given [element], or [ERROR] if the element is
  /// `null`. This is a utility method that can reduce the need for null checks
  /// in other places.
  static ElementKind of(Element element) {
    if (element == null) {
      return ERROR;
    }
    return element.kind;
  }
}

/// The location of an element within the element model.
///
/// Clients may not extend, implement or mix-in this class.
abstract class ElementLocation {
  /// Return the path to the element whose location is represented by this
  /// object. Clients must not modify the returned array.
  List<String> get components;

  /// Return an encoded representation of this location that can be used to
  /// create a location that is equal to this location.
  String get encoding;
}

/// An object that can be used to visit an element structure.
///
/// Clients may not extend, implement or mix-in this class. There are classes
/// that implement this interface that provide useful default behaviors in
/// `package:analyzer/dart/element/visitor.dart`. A couple of the most useful
/// include
/// * SimpleElementVisitor which implements every visit method by doing nothing,
/// * RecursiveElementVisitor which will cause every node in a structure to be
///   visited, and
/// * ThrowingElementVisitor which implements every visit method by throwing an
///   exception.
abstract class ElementVisitor<R> {
  R visitClassElement(ClassElement element);

  R visitCompilationUnitElement(CompilationUnitElement element);

  R visitConstructorElement(ConstructorElement element);

  R visitExportElement(ExportElement element);

  R visitExtensionElement(ExtensionElement element);

  R visitFieldElement(FieldElement element);

  R visitFieldFormalParameterElement(FieldFormalParameterElement element);

  R visitFunctionElement(FunctionElement element);

  R visitFunctionTypeAliasElement(FunctionTypeAliasElement element);

  R visitGenericFunctionTypeElement(GenericFunctionTypeElement element);

  R visitImportElement(ImportElement element);

  R visitLabelElement(LabelElement element);

  R visitLibraryElement(LibraryElement element);

  R visitLocalVariableElement(LocalVariableElement element);

  R visitMethodElement(MethodElement element);

  R visitMultiplyDefinedElement(MultiplyDefinedElement element);

  R visitParameterElement(ParameterElement element);

  R visitPrefixElement(PrefixElement element);

  R visitPropertyAccessorElement(PropertyAccessorElement element);

  R visitTopLevelVariableElement(TopLevelVariableElement element);

  R visitTypeParameterElement(TypeParameterElement element);
}

/// An element representing an executable object, including functions, methods,
/// constructors, getters, and setters.
///
/// Clients may not extend, implement or mix-in this class.
abstract class ExecutableElement implements FunctionTypedElement {
  @override
  ExecutableElement get declaration;

  /// Return `true` if this executable element did not have an explicit return
  /// type specified for it in the original source. Note that if there was no
  /// explicit return type, and if the element model is fully populated, then
  /// the [returnType] will not be `null`.
  bool get hasImplicitReturnType;

  /// Return `true` if this executable element is abstract. Executable elements
  /// are abstract if they are not external and have no body.
  bool get isAbstract;

  /// Return `true` if this executable element has body marked as being
  /// asynchronous.
  bool get isAsynchronous;

  /// Return `true` if this executable element is external. Executable elements
  /// are external if they are explicitly marked as such using the 'external'
  /// keyword.
  bool get isExternal;

  /// Return `true` if this executable element has a body marked as being a
  /// generator.
  bool get isGenerator;

  /// Return `true` if this executable element is an operator. The test may be
  /// based on the name of the executable element, in which case the result will
  /// be correct when the name is legal.
  bool get isOperator;

  /// Return `true` if this element is a static element. A static element is an
  /// element that is not associated with a particular instance, but rather with
  /// an entire library or class.
  bool get isStatic;

  /// Return `true` if this executable element has a body marked as being
  /// synchronous.
  bool get isSynchronous;
}

/// An export directive within a library.
///
/// Clients may not extend, implement or mix-in this class.
abstract class ExportElement implements Element, UriReferencedElement {
  /// Return a list containing the combinators that were specified as part of
  /// the export directive in the order in which they were specified.
  List<NamespaceCombinator> get combinators;

  /// Return the library that is exported from this library by this export
  /// directive, or `null` if the URI has invalid syntax or cannot be resolved.
  LibraryElement get exportedLibrary;
}

/// An element that represents an extension.
///
/// Clients may not extend, implement or mix-in this class.
abstract class ExtensionElement implements TypeParameterizedElement {
  /// Return a list containing all of the accessors (getters and setters)
  /// declared in this extension.
  List<PropertyAccessorElement> get accessors;

  /// Return the type that is extended by this extension.
  DartType get extendedType;

  /// Return a list containing all of the fields declared in this extension.
  List<FieldElement> get fields;

  /// Return a list containing all of the methods declared in this extension.
  List<MethodElement> get methods;

  /// Return the element representing the getter with the given [name] that is
  /// declared in this extension, or `null` if this extension does not declare a
  /// getter with the given name.
  PropertyAccessorElement /*?*/ getGetter(String name);

  /// Return the element representing the method with the given [name] that is
  /// declared in this extension, or `null` if this extension does not declare a
  /// method with the given name.
  MethodElement /*?*/ getMethod(String name);

  /// Return the element representing the setter with the given [name] that is
  /// declared in this extension, or `null` if this extension does not declare a
  /// setter with the given name.
  PropertyAccessorElement /*?*/ getSetter(String name);
}

/// A field defined within a class.
///
/// When the 'extension-methods' experiment is enabled, these elements can also
/// be contained within an extension element.
///
/// Clients may not extend, implement or mix-in this class.
abstract class FieldElement
    implements ClassMemberElement, PropertyInducingElement {
  @override
  FieldElement get declaration;

  /// Return `true` if this field was explicitly marked as being covariant.
  bool get isCovariant;

  /// Return `true` if this element is an enum constant.
  bool get isEnumConstant;

  /// Return `true` if this element is a static element. A static element is an
  /// element that is not associated with a particular instance, but rather with
  /// an entire library or class.
  @override
  bool get isStatic;
}

/// A field formal parameter defined within a constructor element.
///
/// Clients may not extend, implement or mix-in this class.
abstract class FieldFormalParameterElement implements ParameterElement {
  /// Return the field element associated with this field formal parameter, or
  /// `null` if the parameter references a field that doesn't exist.
  FieldElement get field;
}

/// A (non-method) function. This can be either a top-level function, a local
/// function, a closure, or the initialization expression for a field or
/// variable.
///
/// Clients may not extend, implement or mix-in this class.
abstract class FunctionElement implements ExecutableElement, LocalElement {
  /// The name of the method that can be implemented by a class to allow its
  /// instances to be invoked as if they were a function.
  static final String CALL_METHOD_NAME = "call";

  /// The name of the synthetic function defined for libraries that are
  /// deferred.
  static final String LOAD_LIBRARY_NAME = "loadLibrary";

  /// The name of the function used as an entry point.
  static const String MAIN_FUNCTION_NAME = "main";

  /// The name of the method that will be invoked if an attempt is made to
  /// invoke an undefined method on an object.
  static final String NO_SUCH_METHOD_METHOD_NAME = "noSuchMethod";

  /// Return `true` if the function is an entry point, i.e. a top-level function
  /// and has the name `main`.
  bool get isEntryPoint;
}

/// A function type alias (`typedef`).
///
/// Clients may not extend, implement or mix-in this class.
abstract class FunctionTypeAliasElement
    implements FunctionTypedElement, TypeDefiningElement {
  @override
  CompilationUnitElement get enclosingElement;

  /// Return the generic function type element representing the generic function
  /// type on the right side of the equals.
  GenericFunctionTypeElement get function;

  /// Produces the function type resulting from instantiating this typedef with
  /// the given [typeArguments] and [nullabilitySuffix].
  ///
  /// Note that this always instantiates the typedef itself, so for a
  /// [GenericTypeAliasElement] the returned [FunctionType] might still be a
  /// generic function, with type formals. For example, if the typedef is:
  ///     typedef F<T> = void Function<U>(T, U);
  /// then `F<int>` will produce `void Function<U>(int, U)`.
  FunctionType instantiate({
    @required List<DartType> typeArguments,
    @required NullabilitySuffix nullabilitySuffix,
  });

  /// Produces the function type resulting from instantiating this typedef with
  /// the given [typeArguments] and [nullabilitySuffix].
  ///
  /// Note that this always instantiates the typedef itself, so for a
  /// [GenericTypeAliasElement] the returned [FunctionType] might still be a
  /// generic function, with type formals. For example, if the typedef is:
  ///     typedef F<T> = void Function<U>(T, U);
  /// then `F<int>` will produce `void Function<U>(int, U)`.
  @Deprecated('Use instantiate() instead')
  FunctionType instantiate2({
    @required List<DartType> typeArguments,
    @required NullabilitySuffix nullabilitySuffix,
  });

  /// Produces the function type resulting from instantiating this typedef with
  /// type arguments that correspond to the bounds of the type parameters, and
  /// the given [nullabilitySuffix].
  ///
  /// Note that this always instantiates the typedef itself, so for a
  /// [GenericTypeAliasElement] the returned [FunctionType] might still be a
  /// generic function, with type formals. For example, if the typedef is:
  ///     typedef F<T> = void Function<U>(T, U);
  /// then `F<int>` will produce `void Function<U>(int, U)`.
  @Deprecated('Use TypeSystem.instantiateToBounds2() instead')
  FunctionType instantiateToBounds({
    @required NullabilitySuffix nullabilitySuffix,
  });
}

/// An element that has a [FunctionType] as its [type].
///
/// This also provides convenient access to the parameters and return type.
///
/// Clients may not extend, implement or mix-in this class.
abstract class FunctionTypedElement implements TypeParameterizedElement {
  /// Return a list containing all of the parameters defined by this executable
  /// element.
  List<ParameterElement> get parameters;

  /// Return the return type defined by this element. If the element model is
  /// fully populated, then the [returnType] will not be `null`, even if no
  /// return type was explicitly specified.
  DartType get returnType;

  /// Return the type defined by this element.
  FunctionType get type;
}

/// The pseudo-declaration that defines a generic function type.
///
/// Clients may not extend, implement, or mix-in this class.
abstract class GenericFunctionTypeElement implements FunctionTypedElement {}

/// A synonym for [FunctionTypeAliasElement].
///
/// Clients may not extend, implement, or mix-in this class.
abstract class GenericTypeAliasElement implements FunctionTypeAliasElement {}

/// A combinator that causes some of the names in a namespace to be hidden when
/// being imported.
///
/// Clients may not extend, implement or mix-in this class.
abstract class HideElementCombinator implements NamespaceCombinator {
  /// Return a list containing the names that are not to be made visible in the
  /// importing library even if they are defined in the imported library.
  List<String> get hiddenNames;
}

/// A single import directive within a library.
///
/// Clients may not extend, implement or mix-in this class.
abstract class ImportElement implements Element, UriReferencedElement {
  /// Return a list containing the combinators that were specified as part of
  /// the import directive in the order in which they were specified.
  List<NamespaceCombinator> get combinators;

  /// Return the library that is imported into this library by this import
  /// directive, or `null` if the URI has invalid syntax or cannot be resolved.
  LibraryElement get importedLibrary;

  /// Return `true` if this import is for a deferred library.
  bool get isDeferred;

  /// The [Namespace] that this directive contributes to the containing library.
  Namespace get namespace;

  /// Return the prefix that was specified as part of the import directive, or
  /// `null` if there was no prefix specified.
  PrefixElement get prefix;

  /// Return the offset of the prefix of this import in the file that contains
  /// this import directive, or `-1` if this import is synthetic, does not have
  /// a prefix, or otherwise does not have an offset.
  int get prefixOffset;
}

/// A label associated with a statement.
///
/// Clients may not extend, implement or mix-in this class.
abstract class LabelElement implements Element {
  @override
  ExecutableElement get enclosingElement;
}

/// A library.
///
/// Clients may not extend, implement or mix-in this class.
abstract class LibraryElement implements Element {
  /// Return the compilation unit that defines this library.
  CompilationUnitElement get definingCompilationUnit;

  /// Return the entry point for this library, or `null` if this library does
  /// not have an entry point. The entry point is defined to be a zero argument
  /// top-level function whose name is `main`.
  FunctionElement get entryPoint;

  /// Return a list containing all of the libraries that are exported from this
  /// library.
  List<LibraryElement> get exportedLibraries;

  /// The export [Namespace] of this library, `null` if it has not been
  /// computed yet.
  Namespace get exportNamespace;

  /// Return a list containing all of the exports defined in this library.
  List<ExportElement> get exports;

  /// Return `true` if the defining compilation unit of this library contains at
  /// least one import directive whose URI uses the "dart-ext" scheme.
  bool get hasExtUri;

  /// Return `true` if this library defines a top-level function named
  /// `loadLibrary`.
  bool get hasLoadLibraryFunction;

  /// Return an identifier that uniquely identifies this element among the
  /// children of this element's parent.
  String get identifier;

  /// Return a list containing all of the libraries that are imported into this
  /// library. This includes all of the libraries that are imported using a
  /// prefix (also available through the prefixes returned by [getPrefixes]) and
  /// those that are imported without a prefix.
  List<LibraryElement> get importedLibraries;

  /// Return a list containing all of the imports defined in this library.
  List<ImportElement> get imports;

  /// Return `true` if this library is an application that can be run in the
  /// browser.
  bool get isBrowserApplication;

  /// Return `true` if this library is the dart:async library.
  bool get isDartAsync;

  /// Return `true` if this library is the dart:core library.
  bool get isDartCore;

  /// Return `true` if this library is part of the SDK.
  bool get isInSdk;

  bool get isNonNullableByDefault;

  /// The language version for this library.
  LibraryLanguageVersion get languageVersion;

  /// The major component of the language version for this library.
  @Deprecated("Use 'languageVersion'")
  int get languageVersionMajor;

  /// The minor component of the language version for this library.
  @Deprecated("Use 'languageVersion'")
  int get languageVersionMinor;

  /// Return the element representing the synthetic function `loadLibrary` that
  /// is implicitly defined for this library if the library is imported using a
  /// deferred import.
  FunctionElement get loadLibraryFunction;

  /// Return a list containing all of the compilation units that are included in
  /// this library using a `part` directive. This does not include the defining
  /// compilation unit that contains the `part` directives.
  List<CompilationUnitElement> get parts;

  /// Return a list containing elements for each of the prefixes used to
  /// `import` libraries into this library. Each prefix can be used in more
  /// than one `import` directive.
  List<PrefixElement> get prefixes;

  /// The public [Namespace] of this library, `null` if it has not been
  /// computed yet.
  Namespace get publicNamespace;

  /// Return the top-level elements defined in each of the compilation units
  /// that are included in this library. This includes both public and private
  /// elements, but does not include imports, exports, or synthetic elements.
  Iterable<Element> get topLevelElements;

  /// Return the [TypeProvider] that is used in this library.
  TypeProvider get typeProvider;

  /// Return the [TypeSystem] that is used in this library.
  TypeSystem get typeSystem;

  /// Return a list containing all of the compilation units this library
  /// consists of. This includes the defining compilation unit and units
  /// included using the `part` directive.
  List<CompilationUnitElement> get units;

  /// Return a list containing all of the imports that share the given [prefix],
  /// or an empty array if there are no such imports.
  List<ImportElement> getImportsWithPrefix(PrefixElement prefix);

  /// Return the class defined in this library that has the given [name], or
  /// `null` if this library does not define a class with the given name.
  ClassElement getType(String className);

  /// If a legacy library, return the legacy view on the [element].
  /// Otherwise, return the original element.
  T toLegacyElementIfOptOut<T extends Element>(T element);

  /// If a legacy library, return the legacy version of the [type].
  /// Otherwise, return the original type.
  DartType toLegacyTypeIfOptOut(DartType type);
}

class LibraryLanguageVersion {
  /// The version for the whole package that contains this library.
  final Version package;

  /// The version specified using `@dart` override, `null` if absent or invalid.
  final Version override;

  LibraryLanguageVersion({
    @required this.package,
    @required this.override,
  });

  /// The effective language version for the library.
  Version get effective {
    return override ?? package;
  }
}

/// An element that can be (but is not required to be) defined within a method
/// or function (an [ExecutableElement]).
///
/// Clients may not extend, implement or mix-in this class.
abstract class LocalElement implements Element {}

/// A local variable.
///
/// Clients may not extend, implement or mix-in this class.
abstract class LocalVariableElement implements PromotableElement {}

/// An element that represents a method defined within a class.
///
/// When the 'extension-methods' experiment is enabled, these elements can also
/// be contained within an extension element.
///
/// Clients may not extend, implement or mix-in this class.
abstract class MethodElement implements ClassMemberElement, ExecutableElement {
  @override
  MethodElement get declaration;
}

/// A pseudo-element that represents multiple elements defined within a single
/// scope that have the same name. This situation is not allowed by the
/// language, so objects implementing this interface always represent an error.
/// As a result, most of the normal operations on elements do not make sense
/// and will return useless results.
///
/// Clients may not extend, implement or mix-in this class.
abstract class MultiplyDefinedElement implements Element {
  /// Return a list containing all of the elements that were defined within the
  /// scope to have the same name.
  List<Element> get conflictingElements;

  /// Return the type of this element as the dynamic type.
  @deprecated
  DartType get type;
}

/// An [ExecutableElement], with the additional information of a list of
/// [ExecutableElement]s from which this element was composed.
///
/// Clients may not extend, implement or mix-in this class.
abstract class MultiplyInheritedExecutableElement implements ExecutableElement {
  /// Return a list containing all of the executable elements defined within
  /// this executable element.
  List<ExecutableElement> get inheritedElements;
}

/// An object that controls how namespaces are combined.
///
/// Clients may not extend, implement or mix-in this class.
abstract class NamespaceCombinator {}

/// A parameter defined within an executable element.
///
/// Clients may not extend, implement or mix-in this class.
abstract class ParameterElement
    implements PromotableElement, ConstantEvaluationTarget {
  @override
  ParameterElement get declaration;

  /// Return the Dart code of the default value, or `null` if no default value.
  String get defaultValueCode;

  /// Return `true` if this parameter is covariant, meaning it is allowed to
  /// have a narrower type in an override.
  bool get isCovariant;

  /// Return `true` if this parameter is an initializing formal parameter.
  bool get isInitializingFormal;

  /// Return `true` if this parameter is a named parameter. Named parameters
  /// that are annotated with the `@required` annotation are considered
  /// optional.  Named parameters that are annotated with the `required` syntax
  /// are considered required.
  bool get isNamed;

  /// Return `true` if this parameter is a required parameter. Required
  /// parameters are always positional, unless the experiment 'non-nullable' is
  /// enabled, in which case named parameters can also be required.
  ///
  /// Note: regardless of the state of the 'non-nullable' experiment, this will
  /// return `false` for a named parameter that is annotated with the
  /// `@required` annotation.
  // TODO(brianwilkerson) Rename this to `isRequired`.
  bool get isNotOptional;

  /// Return `true` if this parameter is an optional parameter. Optional
  /// parameters can either be positional or named.  Named parameters that are
  /// annotated with the `@required` annotation are considered optional.  Named
  /// parameters that are annotated with the `required` syntax are considered
  /// required.
  bool get isOptional;

  /// Return `true` if this parameter is both an optional and named parameter.
  /// Named parameters that are annotated with the `@required` annotation are
  /// considered optional.  Named parameters that are annotated with the
  /// `required` syntax are considered required.
  bool get isOptionalNamed;

  /// Return `true` if this parameter is both an optional and positional
  /// parameter.
  bool get isOptionalPositional;

  /// Return `true` if this parameter is a positional parameter. Positional
  /// parameters can either be required or optional.
  bool get isPositional;

  /// Return `true` if this parameter is both a required and named parameter.
  /// Named parameters that are annotated with the `@required` annotation are
  /// considered optional.  Named parameters that are annotated with the
  /// `required` syntax are considered required.
  bool get isRequiredNamed;

  /// Return `true` if this parameter is both a required and positional
  /// parameter.
  bool get isRequiredPositional;

  /// Return the kind of this parameter.
  @Deprecated('Use the getters isOptionalNamed, isOptionalPositional, '
      'isRequiredNamed, and isRequiredPositional')
  ParameterKind get parameterKind;

  /// Return a list containing all of the parameters defined by this parameter.
  /// A parameter will only define other parameters if it is a function typed
  /// parameter.
  List<ParameterElement> get parameters;

  /// Return a list containing all of the type parameters defined by this
  /// parameter. A parameter will only define other parameters if it is a
  /// function typed parameter.
  List<TypeParameterElement> get typeParameters;

  /// Append the type, name and possibly the default value of this parameter to
  /// the given [buffer].
  void appendToWithoutDelimiters(
    StringBuffer buffer, {
    bool withNullability = false,
  });
}

/// A prefix used to import one or more libraries into another library.
///
/// Clients may not extend, implement or mix-in this class.
abstract class PrefixElement implements Element {
  @override
  LibraryElement get enclosingElement;
}

/// A variable that might be subject to type promotion.  This might be a local
/// variable or a parameter.
///
/// Clients may not extend, implement or mix-in this class.
abstract class PromotableElement implements LocalElement, VariableElement {}

/// A getter or a setter. Note that explicitly defined property accessors
/// implicitly define a synthetic field. Symmetrically, synthetic accessors are
/// implicitly created for explicitly defined fields. The following rules apply:
///
/// * Every explicit field is represented by a non-synthetic [FieldElement].
/// * Every explicit field induces a getter and possibly a setter, both of which
///   are represented by synthetic [PropertyAccessorElement]s.
/// * Every explicit getter or setter is represented by a non-synthetic
///   [PropertyAccessorElement].
/// * Every explicit getter or setter (or pair thereof if they have the same
///   name) induces a field that is represented by a synthetic [FieldElement].
///
/// Clients may not extend, implement or mix-in this class.
abstract class PropertyAccessorElement implements ExecutableElement {
  /// Return the accessor representing the getter that corresponds to (has the
  /// same name as) this setter, or `null` if this accessor is not a setter or
  /// if there is no corresponding getter.
  PropertyAccessorElement get correspondingGetter;

  /// Return the accessor representing the setter that corresponds to (has the
  /// same name as) this getter, or `null` if this accessor is not a getter or
  /// if there is no corresponding setter.
  PropertyAccessorElement get correspondingSetter;

  @override
  PropertyAccessorElement get declaration;

  /// Return `true` if this accessor represents a getter.
  bool get isGetter;

  /// Return `true` if this accessor represents a setter.
  bool get isSetter;

  /// Return the field or top-level variable associated with this accessor. If
  /// this accessor was explicitly defined (is not synthetic) then the variable
  /// associated with it will be synthetic.
  PropertyInducingElement get variable;
}

/// A variable that has an associated getter and possibly a setter. Note that
/// explicitly defined variables implicitly define a synthetic getter and that
/// non-`final` explicitly defined variables implicitly define a synthetic
/// setter. Symmetrically, synthetic fields are implicitly created for
/// explicitly defined getters and setters. The following rules apply:
///
/// * Every explicit variable is represented by a non-synthetic
///   [PropertyInducingElement].
/// * Every explicit variable induces a getter and possibly a setter, both of
///   which are represented by synthetic [PropertyAccessorElement]s.
/// * Every explicit getter or setter is represented by a non-synthetic
///   [PropertyAccessorElement].
/// * Every explicit getter or setter (or pair thereof if they have the same
///   name) induces a variable that is represented by a synthetic
///   [PropertyInducingElement].
///
/// Clients may not extend, implement or mix-in this class.
abstract class PropertyInducingElement implements VariableElement {
  /// Return the getter associated with this variable. If this variable was
  /// explicitly defined (is not synthetic) then the getter associated with it
  /// will be synthetic.
  PropertyAccessorElement get getter;

  /// Return the setter associated with this variable, or `null` if the variable
  /// is effectively `final` and therefore does not have a setter associated
  /// with it. (This can happen either because the variable is explicitly
  /// defined as being `final` or because the variable is induced by an
  /// explicit getter that does not have a corresponding setter.) If this
  /// variable was explicitly defined (is not synthetic) then the setter
  /// associated with it will be synthetic.
  PropertyAccessorElement get setter;
}

/// A combinator that cause some of the names in a namespace to be visible (and
/// the rest hidden) when being imported.
///
/// Clients may not extend, implement or mix-in this class.
abstract class ShowElementCombinator implements NamespaceCombinator {
  /// Return the offset of the character immediately following the last
  /// character of this node.
  int get end;

  /// Return the offset of the 'show' keyword of this element.
  int get offset;

  /// Return a list containing the names that are to be made visible in the
  /// importing library if they are defined in the imported library.
  List<String> get shownNames;
}

/// A top-level variable.
///
/// Clients may not extend, implement or mix-in this class.
abstract class TopLevelVariableElement implements PropertyInducingElement {
  @override
  TopLevelVariableElement get declaration;
}

/// An element that defines a type.
///
/// Clients may not extend, implement or mix-in this class.
abstract class TypeDefiningElement implements Element {
  /// Return the type defined by this element.
  @deprecated
  DartType get type;
}

/// A type parameter.
///
/// Clients may not extend, implement or mix-in this class.
abstract class TypeParameterElement implements TypeDefiningElement {
  /// Return the type representing the bound associated with this parameter, or
  /// `null` if this parameter does not have an explicit bound. Being able to
  /// distinguish between an implicit and explicit bound is needed by the
  /// instantiate to bounds algorithm.
  DartType get bound;

  @override
  TypeParameterElement get declaration;

  @override
  @deprecated
  TypeParameterType get type;

  /// Create the [TypeParameterType] with the given [nullabilitySuffix] for
  /// this type parameter.
  TypeParameterType instantiate({
    @required NullabilitySuffix nullabilitySuffix,
  });
}

/// An element that has type parameters, such as a class or a typedef. This also
/// includes functions and methods if support for generic methods is enabled.
///
/// Clients may not extend, implement or mix-in this class.
abstract class TypeParameterizedElement implements Element {
  /// If the element defines a type, indicates whether the type may safely
  /// appear without explicit type parameters as the bounds of a type parameter
  /// declaration.
  ///
  /// If the element does not define a type, returns `true`.
  bool get isSimplyBounded;

  /// Return a list containing all of the type parameters declared by this
  /// element directly. This does not include type parameters that are declared
  /// by any enclosing elements.
  List<TypeParameterElement> get typeParameters;
}

/// A pseudo-elements that represents names that are undefined. This situation
/// is not allowed by the language, so objects implementing this interface
/// always represent an error. As a result, most of the normal operations on
/// elements do not make sense and will return useless results.
///
/// Clients may not extend, implement or mix-in this class.
abstract class UndefinedElement implements Element {}

/// An element included into a library using some URI.
///
/// Clients may not extend, implement or mix-in this class.
abstract class UriReferencedElement implements Element {
  /// Return the URI that is used to include this element into the enclosing
  /// library, or `null` if this is the defining compilation unit of a library.
  String get uri;

  /// Return the offset of the character immediately following the last
  /// character of this node's URI, or `-1` for synthetic import.
  int get uriEnd;

  /// Return the offset of the URI in the file, or `-1` if this element is
  /// synthetic.
  int get uriOffset;
}

/// A variable. There are more specific subclasses for more specific kinds of
/// variables.
///
/// Clients may not extend, implement or mix-in this class.
abstract class VariableElement implements Element, ConstantEvaluationTarget {
  /// Return a representation of the value of this variable.
  ///
  /// Return `null` if either this variable was not declared with the 'const'
  /// modifier or if the value of this variable could not be computed because of
  /// errors.
  @Deprecated('Use computeConstantValue() instead')
  DartObject get constantValue;

  @override
  VariableElement get declaration;

  /// Return `true` if this variable element did not have an explicit type
  /// specified for it.
  bool get hasImplicitType;

  /// Return a synthetic function representing this variable's initializer, or
  /// `null` if this variable does not have an initializer. The function will
  /// have no parameters. The return type of the function will be the
  /// compile-time type of the initialization expression.
  FunctionElement get initializer;

  /// Return `true` if this variable was declared with the 'const' modifier.
  bool get isConst;

  /// Return `true` if this variable was declared with the 'final' modifier.
  /// Variables that are declared with the 'const' modifier will return `false`
  /// even though they are implicitly final.
  bool get isFinal;

  /// Return `true` if this variable uses late evaluation semantics.
  ///
  /// This will always return `false` unless the experiment 'non-nullable' is
  /// enabled.
  @experimental
  bool get isLate;

  /// Return `true` if this element is a static variable, as per section 8 of
  /// the Dart Language Specification:
  ///
  /// > A static variable is a variable that is not associated with a particular
  /// > instance, but rather with an entire library or class. Static variables
  /// > include library variables and class variables. Class variables are
  /// > variables whose declaration is immediately nested inside a class
  /// > declaration and includes the modifier static. A library variable is
  /// > implicitly static.
  bool get isStatic;

  /// Return the declared type of this variable, or `null` if the variable did
  /// not have a declared type (such as if it was declared using the keyword
  /// 'var').
  DartType get type;

  /// Return a representation of the value of this variable, forcing the value
  /// to be computed if it had not previously been computed, or `null` if either
  /// this variable was not declared with the 'const' modifier or if the value
  /// of this variable could not be computed because of errors.
  DartObject computeConstantValue();
}