Removed Polymer from Observatory deps

packages/polymer_expressions/lib/eval.dart

Index: packages/polymer_expressions/lib/eval.dart
diff --git a/packages/polymer_expressions/lib/eval.dart b/packages/polymer_expressions/lib/eval.dart
deleted file mode 100644
index 8f7988b662808aac3ea97ceeb8aeeab37d122729..0000000000000000000000000000000000000000
--- a/packages/polymer_expressions/lib/eval.dart
+++ /dev/null
@@ -1,823 +0,0 @@
-// Copyright (c) 2013, 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 polymer_expressions.eval;
-
-import 'dart:async';
-import 'dart:collection';
-
-import 'package:observe/observe.dart';
-import 'package:smoke/smoke.dart' as smoke;
-
-import 'async.dart';
-import 'expression.dart';
-import 'filter.dart';
-import 'visitor.dart';
-
-final _BINARY_OPERATORS = {
-  '+':   (a, b) => a + b,
-  '-':   (a, b) => a - b,
-  '*':   (a, b) => a * b,
-  '/':   (a, b) => a / b,
-  '%':   (a, b) => a % b,
-  '==':  (a, b) => a == b,
-  '!=':  (a, b) => a != b,
-  '===': (a, b) => identical(a, b),
-  '!==': (a, b) => !identical(a, b),
-  '>':   (a, b) => a > b,
-  '>=':  (a, b) => a >= b,
-  '<':   (a, b) => a < b,
-  '<=':  (a, b) => a <= b,
-  '||':  (a, b) => a || b,
-  '&&':  (a, b) => a && b,
-  '|':   (a, f) {
-    if (f is Transformer) return f.forward(a);
-    if (f is Filter) return f(a);
-    throw new EvalException("Filters must be a one-argument function.");
-  }
-};
-
-final _UNARY_OPERATORS = {
-  '+': (a) => a,
-  '-': (a) => -a,
-  '!': (a) => !a,
-};
-
-final _BOOLEAN_OPERATORS = ['!', '||', '&&'];
-
-/**
- * Evaluation [expr] in the context of [scope].
- */
-Object eval(Expression expr, Scope scope) => new EvalVisitor(scope).visit(expr);
-
-/**
- * Returns an [ExpressionObserver] that evaluates [expr] in the context of
- * scope] and listens for any changes on [Observable] values that are
- * returned from sub-expressions. When a value changes the expression is
- * reevaluated and the new result is sent to the [onUpdate] stream of the
- * [ExpressionObsserver].
- */
-ExpressionObserver observe(Expression expr, Scope scope) {
-  var observer = new ObserverBuilder().visit(expr);
-  return observer;
-}
-
-/**
- * Causes [expr] to be reevaluated a returns it's value.
- */
-Object update(ExpressionObserver expr, Scope scope, {skipChanges: false}) {
-  new Updater(scope, skipChanges).visit(expr);
-  return expr.currentValue;
-}
-
-/**
- * Assign [value] to the variable or field referenced by [expr] in the context
- * of [scope].
- *
- * [expr] must be an /assignable/ expression, it must not contain
- * operators or function invocations, and any index operations must use a
- * literal index.
- */
-Object assign(Expression expr, Object value, Scope scope,
-    {bool checkAssignability: true}) {
-
-  Expression expression;
-  var property;
-  bool isIndex = false;
-  var filters = <Expression>[]; // reversed order for assignment
-
-  while (expr is BinaryOperator) {
-    BinaryOperator op = expr;
-    if (op.operator != '|') {
-      break;
-    }
-    filters.add(op.right);
-    expr = op.left;
-  }
-
-  if (expr is Identifier) {
-    expression = empty();
-    property = expr.value;
-  } else if (expr is Index) {
-    expression = expr.receiver;
-    property = expr.argument;
-    isIndex = true;
-  } else if (expr is Getter) {
-    expression = expr.receiver;
-    property = expr.name;
-  } else {
-    if (checkAssignability) {
-      throw new EvalException("Expression is not assignable: $expr");
-    }
-    return null;
-  }
-
-  // transform the values backwards through the filters
-  for (var filterExpr in filters) {
-    var filter = eval(filterExpr, scope);
-    if (filter is! Transformer) {
-      if (checkAssignability) {
-        throw new EvalException("filter must implement Transformer to be "
-            "assignable: $filterExpr");
-      } else {
-        return null;
-      }
-    }
-    value = filter.reverse(value);
-  }
-  // evaluate the receiver
-  var o = eval(expression, scope);
-
-  // can't assign to a property on a null LHS object. Silently fail.
-  if (o == null) return null;
-
-  if (isIndex) {
-    var index = eval(property, scope);
-    o[index] = value;
-  } else {
-    smoke.write(o, smoke.nameToSymbol(property), value);
-  }
-  return value;
-}
-
-
-/**
- * A scope in polymer expressions that can map names to objects. Scopes contain
- * a set of named variables and a unique model object. The scope structure
- * is then used to lookup names using the `[]` operator. The lookup first
- * searches for the name in local variables, then in global variables,
- * and then finally looks up the name as a property in the model.
- */
-abstract class Scope implements Indexable<String, Object> {
-  Scope._();
-
-  /** Create a scope containing a [model] and all of [variables]. */
-  factory Scope({Object model, Map<String, Object> variables}) {
-    var scope = new _ModelScope(model);
-    return variables == null ? scope
-        : new _GlobalsScope(new Map<String, Object>.from(variables), scope);
-  }
-
-  /** Return the unique model in this scope. */
-  Object get model;
-
-  /**
-   * Lookup the value of [name] in the current scope. If [name] is 'this', then
-   * we return the [model]. For any other name, this finds the first variable
-   * matching [name] or, if none exists, the property [name] in the [model].
-   */
-  Object operator [](String name);
-
-  operator []=(String name, Object value) {
-    throw new UnsupportedError('[]= is not supported in Scope.');
-  }
-
-  /**
-   * Returns whether [name] is defined in [model], that is, a lookup
-   * would not find a variable with that name, but there is a non-null model
-   * where we can look it up as a property.
-   */
-  bool _isModelProperty(String name);
-
-  /** Create a new scope extending this scope with an additional variable. */
-  Scope childScope(String name, Object value) =>
-      new _LocalVariableScope(name, value, this);
-}
-
-/**
- * A scope that looks up names in a model object. This kind of scope has no
- * parent scope because all our lookup operations stop when we reach the model
- * object. Any variables added in scope or global variables are added as child
- * scopes.
- */
-class _ModelScope extends Scope {
-  final Object model;
-
-  _ModelScope(this.model) : super._();
-
-  Object operator[](String name) {
-    if (name == 'this') return model;
-    var symbol = smoke.nameToSymbol(name);
-    if (model == null || symbol == null) {
-      throw new EvalException("variable '$name' not found");
-    }
-    return _convert(smoke.read(model, symbol));
-  }
-
-  Object _isModelProperty(String name) => name != 'this';
-
-  String toString() => "[model: $model]";
-}
-
-/**
- * A scope that holds a reference to a single variable. Polymer expressions
- * introduce variables to the scope one at a time. Each time a variable is
- * added, a new [_LocalVariableScope] is created.
- */
-class _LocalVariableScope extends Scope {
-  final Scope parent;
-  final String varName;
-  // TODO(sigmund,justinfagnani): make this @observable?
-  final Object value;
-
-  _LocalVariableScope(this.varName, this.value, this.parent) : super._() {
-    if (varName == 'this') {
-      throw new EvalException("'this' cannot be used as a variable name.");
-    }
-  }
-
-  Object get model => parent != null ? parent.model : null;
-
-  Object operator[](String name) {
-    if (varName == name) return _convert(value);
-    if (parent != null) return parent[name];
-    throw new EvalException("variable '$name' not found");
-  }
-
-  bool _isModelProperty(String name) {
-    if (varName == name) return false;
-    return parent == null ? false : parent._isModelProperty(name);
-  }
-
-  String toString() => "$parent > [local: $varName]";
-}
-
-/** A scope that holds a reference to a global variables. */
-class _GlobalsScope extends Scope {
-  final _ModelScope parent;
-  final Map<String, Object> variables;
-
-  _GlobalsScope(this.variables, this.parent) : super._() {
-    if (variables.containsKey('this')) {
-      throw new EvalException("'this' cannot be used as a variable name.");
-    }
-  }
-
-  Object get model => parent != null ? parent.model : null;
-
-  Object operator[](String name) {
-    if (variables.containsKey(name)) return _convert(variables[name]);
-    if (parent != null) return parent[name];
-    throw new EvalException("variable '$name' not found");
-  }
-
-  bool _isModelProperty(String name) {
-    if (variables.containsKey(name)) return false;
-    return parent == null ? false : parent._isModelProperty(name);
-  }
-
-  String toString() => "$parent > [global: ${variables.keys}]";
-}
-
-Object _convert(v) => v is Stream ? new StreamBinding(v) : v;
-
-abstract class ExpressionObserver<E extends Expression> implements Expression {
-  final E _expr;
-  ExpressionObserver _parent;
-
-  StreamSubscription _subscription;
-  Object _value;
-
-  StreamController _controller = new StreamController.broadcast();
-  Stream get onUpdate => _controller.stream;
-
-  ExpressionObserver(this._expr);
-
-  Expression get expression => _expr;
-
-  Object get currentValue => _value;
-
-  update(Scope scope) => _updateSelf(scope);
-
-  _updateSelf(Scope scope) {}
-
-  _invalidate(Scope scope) {
-    _observe(scope, false);
-    if (_parent != null) {
-      _parent._invalidate(scope);
-    }
-  }
-
-  _unobserve() {
-    if (_subscription != null) {
-      _subscription.cancel();
-      _subscription = null;
-    }
-  }
-
-  _observe(Scope scope, skipChanges) {
-    _unobserve();
-
-    var _oldValue = _value;
-
-    // evaluate
-    _updateSelf(scope);
-
-    if (!skipChanges && !identical(_value, _oldValue)) {
-      _controller.add(_value);
-    }
-  }
-
-  String toString() => _expr.toString();
-}
-
-class Updater extends RecursiveVisitor {
-  final Scope scope;
-  final bool skipChanges;
-
-  Updater(this.scope, [this.skipChanges = false]);
-
-  visitExpression(ExpressionObserver e) {
-    e._observe(scope, skipChanges);
-  }
-}
-
-class Closer extends RecursiveVisitor {
-  static final _instance = new Closer._();
-  factory Closer() => _instance;
-  Closer._();
-
-  visitExpression(ExpressionObserver e) {
-    e._unobserve();
-  }
-}
-
-class EvalVisitor extends Visitor {
-  final Scope scope;
-
-  EvalVisitor(this.scope);
-
-  visitEmptyExpression(EmptyExpression e) => scope.model;
-
-  visitParenthesizedExpression(ParenthesizedExpression e) => visit(e.child);
-
-  visitGetter(Getter g) {
-    var receiver = visit(g.receiver);
-    if (receiver == null) return null;
-    var symbol = smoke.nameToSymbol(g.name);
-    return smoke.read(receiver, symbol);
-  }
-
-  visitIndex(Index i) {
-    var receiver = visit(i.receiver);
-    if (receiver == null) return null;
-    var key = visit(i.argument);
-    return receiver[key];
-  }
-
-  visitInvoke(Invoke i) {
-    var receiver = visit(i.receiver);
-    if (receiver == null) return null;
-    var args = (i.arguments == null)
-        ? null
-        : i.arguments.map(visit).toList(growable: false);
-
-    if (i.method == null) {
-      assert(receiver is Function);
-      return Function.apply(receiver, args);
-    }
-
-    var symbol = smoke.nameToSymbol(i.method);
-    return smoke.invoke(receiver, symbol, args);
-  }
-
-  visitLiteral(Literal l) => l.value;
-
-  visitListLiteral(ListLiteral l) => l.items.map(visit).toList();
-
-  visitMapLiteral(MapLiteral l) {
-    var map = {};
-    for (var entry in l.entries) {
-      var key = visit(entry.key);
-      var value = visit(entry.entryValue);
-      map[key] = value;
-    }
-    return map;
-  }
-
-  visitMapLiteralEntry(MapLiteralEntry e) =>
-      throw new UnsupportedError("should never be called");
-
-  visitIdentifier(Identifier i) => scope[i.value];
-
-  visitBinaryOperator(BinaryOperator o) {
-    var operator = o.operator;
-    var left = visit(o.left);
-    var right = visit(o.right);
-
-    var f = _BINARY_OPERATORS[operator];
-    if (operator == '&&' || operator == '||') {
-      // TODO: short-circuit
-      return f(_toBool(left), _toBool(right));
-    } else if (operator == '==' || operator == '!=') {
-      return f(left, right);
-    } else if (left == null || right == null) {
-      return null;
-    }
-    return f(left, right);
-  }
-
-  visitUnaryOperator(UnaryOperator o) {
-    var expr = visit(o.child);
-    var f = _UNARY_OPERATORS[o.operator];
-    if (o.operator == '!') {
-      return f(_toBool(expr));
-    }
-    return (expr == null) ? null : f(expr);
-  }
-
-  visitTernaryOperator(TernaryOperator o) =>
-      visit(o.condition) == true ? visit(o.trueExpr) : visit(o.falseExpr);
-
-  visitInExpression(InExpression i) =>
-      throw new UnsupportedError("can't eval an 'in' expression");
-
-  visitAsExpression(AsExpression i) =>
-      throw new UnsupportedError("can't eval an 'as' expression");
-}
-
-class ObserverBuilder extends Visitor {
-  final Queue parents = new Queue();
-
-  ObserverBuilder();
-
-  visitEmptyExpression(EmptyExpression e) => new EmptyObserver(e);
-
-  visitParenthesizedExpression(ParenthesizedExpression e) => visit(e.child);
-
-  visitGetter(Getter g) {
-    var receiver = visit(g.receiver);
-    var getter = new GetterObserver(g, receiver);
-    receiver._parent = getter;
-    return getter;
-  }
-
-  visitIndex(Index i) {
-    var receiver = visit(i.receiver);
-    var arg = visit(i.argument);
-    var index =  new IndexObserver(i, receiver, arg);
-    receiver._parent = index;
-    arg._parent = index;
-    return index;
-  }
-
-  visitInvoke(Invoke i) {
-    var receiver = visit(i.receiver);
-    var args = (i.arguments == null)
-        ? null
-        : i.arguments.map(visit).toList(growable: false);
-    var invoke =  new InvokeObserver(i, receiver, args);
-    receiver._parent = invoke;
-    if (args != null) args.forEach((a) => a._parent = invoke);
-    return invoke;
-  }
-
-  visitLiteral(Literal l) => new LiteralObserver(l);
-
-  visitListLiteral(ListLiteral l) {
-    var items = l.items.map(visit).toList(growable: false);
-    var list = new ListLiteralObserver(l, items);
-    items.forEach((e) => e._parent = list);
-    return list;
-  }
-
-  visitMapLiteral(MapLiteral l) {
-    var entries = l.entries.map(visit).toList(growable: false);
-    var map = new MapLiteralObserver(l, entries);
-    entries.forEach((e) => e._parent = map);
-    return map;
-  }
-
-  visitMapLiteralEntry(MapLiteralEntry e) {
-    var key = visit(e.key);
-    var value = visit(e.entryValue);
-    var entry = new MapLiteralEntryObserver(e, key, value);
-    key._parent = entry;
-    value._parent = entry;
-    return entry;
-  }
-
-  visitIdentifier(Identifier i) => new IdentifierObserver(i);
-
-  visitBinaryOperator(BinaryOperator o) {
-    var left = visit(o.left);
-    var right = visit(o.right);
-    var binary = new BinaryObserver(o, left, right);
-    left._parent = binary;
-    right._parent = binary;
-    return binary;
-  }
-
-  visitUnaryOperator(UnaryOperator o) {
-    var expr = visit(o.child);
-    var unary = new UnaryObserver(o, expr);
-    expr._parent = unary;
-    return unary;
-  }
-
-  visitTernaryOperator(TernaryOperator o) {
-    var condition = visit(o.condition);
-    var trueExpr = visit(o.trueExpr);
-    var falseExpr = visit(o.falseExpr);
-    var ternary = new TernaryObserver(o, condition, trueExpr, falseExpr);
-    condition._parent = ternary;
-    trueExpr._parent = ternary;
-    falseExpr._parent = ternary;
-    return ternary;
-  }
-
-  visitInExpression(InExpression i) {
-    throw new UnsupportedError("can't eval an 'in' expression");
-  }
-
-  visitAsExpression(AsExpression i) {
-    throw new UnsupportedError("can't eval an 'as' expression");
-  }
-}
-
-class EmptyObserver extends ExpressionObserver<EmptyExpression>
-    implements EmptyExpression {
-
-  EmptyObserver(EmptyExpression value) : super(value);
-
-  _updateSelf(Scope scope) {
-    _value = scope.model;
-    // TODO(justin): listen for scope.model changes?
-  }
-
-  accept(Visitor v) => v.visitEmptyExpression(this);
-}
-
-class LiteralObserver extends ExpressionObserver<Literal> implements Literal {
-
-  LiteralObserver(Literal value) : super(value);
-
-  dynamic get value => _expr.value;
-
-  _updateSelf(Scope scope) {
-    _value = _expr.value;
-  }
-
-  accept(Visitor v) => v.visitLiteral(this);
-}
-
-class ListLiteralObserver extends ExpressionObserver<ListLiteral>
-    implements ListLiteral {
-
-  final List<ExpressionObserver> items;
-
-  ListLiteralObserver(ListLiteral value, this.items) : super(value);
-
-  _updateSelf(Scope scope) {
-    _value = items.map((i) => i._value).toList();
-  }
-
-  accept(Visitor v) => v.visitListLiteral(this);
-}
-
-class MapLiteralObserver extends ExpressionObserver<MapLiteral>
-    implements MapLiteral {
-
-  final List<MapLiteralEntryObserver> entries;
-
-  MapLiteralObserver(MapLiteral value, this.entries) : super(value);
-
-  _updateSelf(Scope scope) {
-    _value = entries.fold(new Map(),
-        (m, e) => m..[e.key._value] = e.entryValue._value);
-  }
-
-  accept(Visitor v) => v.visitMapLiteral(this);
-}
-
-class MapLiteralEntryObserver extends ExpressionObserver<MapLiteralEntry>
-    implements MapLiteralEntry {
-
-  final LiteralObserver key;
-  final ExpressionObserver entryValue;
-
-  MapLiteralEntryObserver(MapLiteralEntry value, this.key, this.entryValue)
-      : super(value);
-
-  accept(Visitor v) => v.visitMapLiteralEntry(this);
-}
-
-class IdentifierObserver extends ExpressionObserver<Identifier>
-    implements Identifier {
-
-  IdentifierObserver(Identifier value) : super(value);
-
-  String get value => _expr.value;
-
-  _updateSelf(Scope scope) {
-    _value = scope[value];
-    if (!scope._isModelProperty(value)) return;
-    var model = scope.model;
-    if (model is! Observable) return;
-    var symbol = smoke.nameToSymbol(value);
-    _subscription = (model as Observable).changes.listen((changes) {
-      if (changes.any((c) => c is PropertyChangeRecord && c.name == symbol)) {
-        _invalidate(scope);
-      }
-    });
-  }
-
-  accept(Visitor v) => v.visitIdentifier(this);
-}
-
-class ParenthesizedObserver extends ExpressionObserver<ParenthesizedExpression>
-    implements ParenthesizedExpression {
-  final ExpressionObserver child;
-
-  ParenthesizedObserver(ParenthesizedExpression expr, this.child) : super(expr);
-
-
-  _updateSelf(Scope scope) {
-    _value = child._value;
-  }
-
-  accept(Visitor v) => v.visitParenthesizedExpression(this);
-}
-
-class UnaryObserver extends ExpressionObserver<UnaryOperator>
-    implements UnaryOperator {
-  final ExpressionObserver child;
-
-  UnaryObserver(UnaryOperator expr, this.child) : super(expr);
-
-  String get operator => _expr.operator;
-
-  _updateSelf(Scope scope) {
-    var f = _UNARY_OPERATORS[_expr.operator];
-    if (operator == '!') {
-      _value = f(_toBool(child._value));
-    } else {
-      _value = (child._value == null) ? null : f(child._value);
-    }
-  }
-
-  accept(Visitor v) => v.visitUnaryOperator(this);
-}
-
-class BinaryObserver extends ExpressionObserver<BinaryOperator>
-    implements BinaryOperator {
-
-  final ExpressionObserver left;
-  final ExpressionObserver right;
-
-  BinaryObserver(BinaryOperator expr, this.left, this.right)
-      : super(expr);
-
-  String get operator => _expr.operator;
-
-  _updateSelf(Scope scope) {
-    var f = _BINARY_OPERATORS[operator];
-    if (operator == '&&' || operator == '||') {
-      _value = f(_toBool(left._value), _toBool(right._value));
-    } else if (operator == '==' || operator == '!=') {
-      _value = f(left._value, right._value);
-    } else if (left._value == null || right._value == null) {
-      _value = null;
-    } else {
-      if (operator == '|' && left._value is ObservableList) {
-        _subscription = (left._value as ObservableList).listChanges
-            .listen((_) => _invalidate(scope));
-      }
-      _value = f(left._value, right._value);
-    }
-  }
-
-  accept(Visitor v) => v.visitBinaryOperator(this);
-
-}
-
-class TernaryObserver extends ExpressionObserver<TernaryOperator>
-    implements TernaryOperator {
-
-  final ExpressionObserver condition;
-  final ExpressionObserver trueExpr;
-  final ExpressionObserver falseExpr;
-
-  TernaryObserver(TernaryOperator expr, this.condition, this.trueExpr,
-      this.falseExpr) : super(expr);
-
-  _updateSelf(Scope scope) {
-    _value = _toBool(condition._value) ? trueExpr._value : falseExpr._value;
-  }
-
-  accept(Visitor v) => v.visitTernaryOperator(this);
-
-}
-
-class GetterObserver extends ExpressionObserver<Getter> implements Getter {
-  final ExpressionObserver receiver;
-
-  GetterObserver(Expression expr, this.receiver) : super(expr);
-
-  String get name => _expr.name;
-
-  _updateSelf(Scope scope) {
-    var receiverValue = receiver._value;
-    if (receiverValue == null) {
-      _value = null;
-      return;
-    }
-    var symbol = smoke.nameToSymbol(_expr.name);
-    _value = smoke.read(receiverValue, symbol);
-
-    if (receiverValue is Observable) {
-      _subscription = (receiverValue as Observable).changes.listen((changes) {
-        if (changes.any((c) => c is PropertyChangeRecord && c.name == symbol)) {
-          _invalidate(scope);
-        }
-      });
-    }
-  }
-
-  accept(Visitor v) => v.visitGetter(this);
-}
-
-class IndexObserver extends ExpressionObserver<Index> implements Index {
-  final ExpressionObserver receiver;
-  final ExpressionObserver argument;
-
-  IndexObserver(Expression expr, this.receiver, this.argument) : super(expr);
-
-  _updateSelf(Scope scope) {
-    var receiverValue = receiver._value;
-    if (receiverValue == null) {
-      _value = null;
-      return;
-    }
-    var key = argument._value;
-    _value = receiverValue[key];
-
-    if (receiverValue is ObservableList) {
-      _subscription = (receiverValue as ObservableList).listChanges
-          .listen((changes) {
-        if (changes.any((c) => c.indexChanged(key))) _invalidate(scope);
-      });
-    } else if (receiverValue is Observable) {
-      _subscription = (receiverValue as Observable).changes.listen((changes) {
-        if (changes.any((c) => c is MapChangeRecord && c.key == key)) {
-          _invalidate(scope);
-        }
-      });
-    }
-  }
-
-  accept(Visitor v) => v.visitIndex(this);
-}
-
-class InvokeObserver extends ExpressionObserver<Invoke> implements Invoke {
-  final ExpressionObserver receiver;
-  final List<ExpressionObserver> arguments;
-
-  InvokeObserver(Expression expr, this.receiver, this.arguments)
-      : super(expr) {
-    assert(arguments != null);
-  }
-
-  String get method => _expr.method;
-
-  _updateSelf(Scope scope) {
-    var args = arguments.map((a) => a._value).toList();
-    var receiverValue = receiver._value;
-    if (receiverValue == null) {
-      _value = null;
-      return;
-    }
-    if (_expr.method == null) {
-      // top-level function or model method
-      // TODO(justin): listen to model changes to see if the method has
-      // changed? listen to the scope to see if the top-level method has
-      // changed?
-      assert(receiverValue is Function);
-      _value = _convert(Function.apply(receiverValue, args));
-    } else {
-      var symbol = smoke.nameToSymbol(_expr.method);
-      _value = smoke.invoke(receiverValue, symbol, args);
-
-      if (receiverValue is Observable) {
-        _subscription = (receiverValue as Observable).changes.listen(
-            (List<ChangeRecord> changes) {
-              if (changes.any(
-                  (c) => c is PropertyChangeRecord && c.name == symbol)) {
-                _invalidate(scope);
-              }
-            });
-      }
-    }
-  }
-
-  accept(Visitor v) => v.visitInvoke(this);
-}
-
-_toBool(v) => (v == null) ? false : v;
-
-class EvalException implements Exception {
-  final String message;
-  EvalException(this.message);
-  String toString() => "EvalException: $message";
-}