Add support for binding 'this'

pkg/kernel/lib/interpreter/interpreter.dart

Index: pkg/kernel/lib/interpreter/interpreter.dart
diff --git a/pkg/kernel/lib/interpreter/interpreter.dart b/pkg/kernel/lib/interpreter/interpreter.dart
index b3787a671c6e2e213a633ce2ee010e808ccde172..f75d0777c5ab0e99e41e4b34b4317e738f473455 100644
--- a/pkg/kernel/lib/interpreter/interpreter.dart
+++ b/pkg/kernel/lib/interpreter/interpreter.dart
@@ -53,16 +53,29 @@ class Environment {
   final List<Binding> bindings = <Binding>[];
   final Environment parent;
 
+  Value get thisInstance {
+    return containsThis()
+        ? lookupThis().value
+        : throw "Invalid reference to 'this' expression";
+  }
+
   Environment.empty() : parent = null;
   Environment(this.parent);
 
   bool contains(VariableDeclaration variable) {
-    for (Binding b in bindings.reversed) {
+    for (Binding b in bindings) {
       if (identical(b.variable, variable)) return true;
     }
     return parent?.contains(variable) ?? false;
   }
 
+  bool containsThis() {
+    for (Binding b in bindings) {
+      if (identical(b.variable.name, 'this')) return true;
+    }
+    return parent?.containsThis() ?? false;
+  }
+
   Binding lookupBinding(VariableDeclaration variable) {
     assert(contains(variable));
     for (Binding b in bindings) {
@@ -71,6 +84,14 @@ class Environment {
     return parent.lookupBinding(variable);
   }
 
+  Location lookupThis() {
+    assert(containsThis());
+    for (Binding b in bindings) {
+      if (identical(b.variable.name, 'this')) return b.location;
+    }
+    return parent.lookupThis();
+  }
+
   Value lookup(VariableDeclaration variable) {
     return lookupBinding(variable).location.value;
   }
@@ -85,6 +106,11 @@ class Environment {
     return new Environment(this)
       ..bindings.add(new Binding(variable, new Location(value)));
   }
+
+  Environment extendWithThis(ObjectValue v) {
+    assert(!containsThis());
+    return extend(new VariableDeclaration('this'), v);
+  }
 }
 
 /// Evaluate expressions.