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..b02709f099a355ea752c8984cd8e805320ccfe21 100644
--- a/pkg/kernel/lib/interpreter/interpreter.dart
+++ b/pkg/kernel/lib/interpreter/interpreter.dart
@@ -53,6 +53,12 @@ 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);
 
@@ -63,6 +69,13 @@ class Environment {
     return parent?.contains(variable) ?? false;
   }
 
+  bool containsThis() {
+    for (Binding b in bindings.reversed) {

[Dmitry Stefantsov, 2017/07/14 00:43:08]

Is it important that [bindings] is reversed?

[zhivkag, 2017/07/14 07:02:31]

Done.

+      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());

[Dmitry Stefantsov, 2017/07/14 00:43:07]

If we pass this `assert`, then at some iteration of the following loop we'll
return some location, and `parent.lookupThis()` will never be executed.

[zhivkag, 2017/07/14 07:02:31]

The intent of this assert is similar to "contains" for normal variables
declarations: check if `this` is bound in the current environment or any of its
parents.
When the assert passes, a location will eventually be returned, but not
necessarily from the loop below, it could also be with `parent.lookupThis()`.

[Dmitry Stefantsov, 2017/07/18 10:45:15]

Yep.  I misinterpreted [containsThis] above.  Thanks for the clarification!

+    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.