Pass in `this` as a free variable to the closure class

pkg/compiler/lib/src/js_model/closure_visitors.dart

Index: pkg/compiler/lib/src/js_model/closure_visitors.dart
diff --git a/pkg/compiler/lib/src/js_model/closure_visitors.dart b/pkg/compiler/lib/src/js_model/closure_visitors.dart
index f27c6dd20a3f1e970714019e6ed4eea99b9c1f7e..c9466cf01a225b4ec05a5d9acc54484e862454ee 100644
--- a/pkg/compiler/lib/src/js_model/closure_visitors.dart
+++ b/pkg/compiler/lib/src/js_model/closure_visitors.dart
@@ -177,6 +177,42 @@ class CapturedScopeBuilder extends ir.Visitor {
     }
   }
 
+  @override
+  void visitThisExpression(ir.ThisExpression thisExpression) {
+    if (_hasThisLocal) _registerNeedsThis();
+  }
+
+  @override
+  void visitTypeParameter(ir.TypeParameter typeParameter) {
+    ir.TreeNode context = _executableContext;
+    if (_isInsideClosure && context is ir.Procedure && context.isFactory) {
+      // This is a closure in a factory constructor.  Since there is no
+      // [:this:], we have to mark the type arguments as free variables to
+      // capture them in the closure.
+      // TODO(efortuna): Implement for in the case of RTI.
+      // useTypeVariableAsLocal(typeParameter.bound);
+    }
+
+    if (_executableContext is ir.Member &&
+        _executableContext is! ir.Field &&
+        _hasThisLocal) {
+      // In checked mode, using a type variable in a type annotation may lead
+      // to a runtime type check that needs to access the type argument and
+      // therefore the closure needs a this-element, if it is not in a field
+      // initializer; field initializers are evaluated in a context where
+      // the type arguments are available in locals.
+      _registerNeedsThis();
+    }
+  }
+
+  /// Add `this` as a variable that needs to be accessed (and thus may become a
+  /// free/captured variable.
+  void _registerNeedsThis() {
+    if (_isInsideClosure) {
+      _currentScopeInfo.freeVariables.add(const ThisVariable());

[Johnni Winther, 2017/08/31 07:24:38]

Add a `needsThis` property on [KernelScopeInfo] instead of adding
[ThisVariable].

[Emily Fortuna, 2017/08/31 17:40:52]

Done.

+    }
+  }
+
   @override
   void visitForStatement(ir.ForStatement node) {
     List<ir.VariableDeclaration> boxedLoopVariables =
@@ -303,3 +339,85 @@ class CapturedScopeBuilder extends ir.Visitor {
     visitInvokable(functionDeclaration);
   }
 }
+
+/// A fake sentinel "ir" node place holder representing the usage of `this`
+/// inside closures that occur inside members.
+class ThisVariable implements ir.VariableDeclaration {
+  const ThisVariable();
+
+  bool get isConst => true;
+  set isConst(bool constVal) =>
+      throw new UnsupportedError("ThisVariable.setIsConst");
+  bool get isFinal => true;
+  set isFinal(bool finalValue) =>
+      throw new UnsupportedError("ThisVariable.setIsFinal");
+  bool get isCovariant => false;
+  set isCovariant(bool covariant) =>
+      throw new UnsupportedError("ThisVariable.setIsCovariant");
+  bool get isFieldFormal => false;
+  set isFieldFormal(bool formal) =>
+      throw new UnsupportedError("ThisVariable.setIsFieldFormal");
+  String get name => 'this';
+  set name(String name) => throw new UnsupportedError("ThisVariable.setName");
+  set type(ir.DartType type) => throw new UnsupportedError("ThisVariable.type");
+  int get fileOffset => throw new UnsupportedError("ThisVariable.fileOffset");
+  set fileOffset(int offset) =>
+      throw new UnsupportedError("ThisVariable.setFileOffset");
+
+  int get binaryOffsetNoTag =>
+      throw new UnsupportedError("ThisVariable.binaryOffsetNoTag");
+  set binaryOffsetNoTag(int offset) =>
+      throw new UnsupportedError("ThisVariable.binaryOffsetNoTag");
+
+  int get fileEqualsOffset =>
+      throw new UnsupportedError("ThisVariable.fileEqualsOffset");
+  set fileEqualsOffset(int offset) =>
+      throw new UnsupportedError("ThisVariable.setFileEqualsOffset");
+
+  int get flags => throw new UnsupportedError("ThisVariable.flags");
+  set flags(int flags) => throw new UnsupportedError("ThisVariable.flags");
+
+  ir.DartType get type =>
+      throw new UnsupportedError("Sentinel this does not have a type.");
+
+  ir.TreeNode get parent =>
+      throw new UnsupportedError("Sentinel this does not have a parent.");
+
+  set parent(ir.TreeNode parent) =>
+      throw new UnsupportedError("Sentinel this does not have a parent.");
+
+  ir.Expression get initializer => null;
+  set initializer(ir.Expression expression) =>
+      throw new UnsupportedError("ThisVariable.setInitializer");
+
+  accept(ir.StatementVisitor v) => throw new UnsupportedError(
+      "ThisVariable should not be walked in AST traversal");
+  accept1(ir.StatementVisitor1 v, arg) => throw new UnsupportedError(
+      "ThisVariable should not be walked in AST traversal");
+
+  visitChildren(ir.Visitor v) {
+    throw new UnsupportedError(
+        "ThisVariable should not be walked in AST traversal");
+  }
+
+  transformChildren(ir.Transformer v) {
+    throw new UnsupportedError(
+        "ThisVariable should not be walked in AST traversal");
+  }
+
+  void replaceChild(ir.TreeNode child, ir.TreeNode replacement) =>
+      throw new UnsupportedError("ThisVariable.replaceChild");
+
+  void replaceWith(ir.TreeNode replacement) =>
+      throw new UnsupportedError("ThisVariable.replaceWith");
+
+  void remove() => throw new UnsupportedError("ThisVariable.remove");
+
+  ir.Program get enclosingProgram =>
+      throw new UnsupportedError("ThisVariable.enclosingProgram");
+
+  ir.Location get location =>
+      throw new UnsupportedError("ThisVariable.location");
+
+  String toString() => 'thisVariable';
+}